ttyA has ld associated to n_gsm, when ttyA is closing, it triggers
to release gsmttyB's ld data dlci[B], then race would happen if gsmttyB
is opening in parallel.

(Note: This patch set differs from previous set in that it uses mutex
instead of spin lock to avoid race, so that it avoids sleeping in automic
context)

Here are race cases we found recently in test:

CASE #1
====================================================================
releasing dlci[B] race with gsmtty_install(gsmttyB), then panic
in gsmtty_open(gsmttyB), as below:

 tty_release(ttyA)                  tty_open(gsmttyB)
     |                                   |
   -----                           gsmtty_install(gsmttyB)
     |                                   |
   -----                    gsm_dlci_alloc(gsmttyB) => alloc dlci[B]
 tty_ldisc_release(ttyA)               -----
     |                                   |
 gsm_dlci_release(dlci[B])             -----
     |                                   |
 gsm_dlci_free(dlci[B])                -----
     |                                   |
   -----                           gsmtty_open(gsmttyB)

 gsmtty_open()
 {
     struct gsm_dlci *dlci = tty->driver_data; => here it uses dlci[B]
     ...
 }

 In gsmtty_open(gsmttyA), it uses dlci[B] which was release, so hit a panic.
=====================================================================

CASE #2
=====================================================================
releasing dlci[0] race with gsmtty_install(gsmttyB), then panic
in gsmtty_open(), as below:

 tty_release(ttyA)                  tty_open(gsmttyB)
     |                                   |
   -----                           gsmtty_install(gsmttyB)
     |                                   |
   -----                    gsm_dlci_alloc(gsmttyB) => alloc dlci[B]
     |                                   |
   -----                         gsmtty_open(gsmttyB) fail
     |                                   |
   -----                           tty_release(gsmttyB)
     |                                   |
   -----                           gsmtty_close(gsmttyB)
     |                                   |
   -----                        gsmtty_detach_dlci(dlci[B])
     |                                   |
   -----                             dlci_put(dlci[B])
     |                                   |
 tty_ldisc_release(ttyA)               -----
     |                                   |
 gsm_dlci_release(dlci[0])             -----
     |                                   |
 gsm_dlci_free(dlci[0])                -----
     |                                   |
   -----                             dlci_put(dlci[0])

 In gsmtty_detach_dlci(dlci[B]), it tries to use dlci[0] which was released,
 then hit panic.
=====================================================================

IMHO, n_gsm tty operations would refer released ldisc,  as long as
gsm_dlci_release() has chance to release ldisc data when some gsmtty operations
are ongoing..

This patch is try to avoid it by:

1) in n_gsm driver, use a global gsm mutex lock to avoid gsm_dlci_release() run in
parallel with gsmtty_install();

2) Increase dlci's ref count in gsmtty_install() instead of in gsmtty_open(), the
purpose is to prevent gsm_dlci_release() releasing dlci after gsmtty_install()
allocats dlci but before gsmtty_open increases dlci's ref count;

3) Decrease dlci's ref count in gsmtty_remove(), a tty framework API, this is the
opposite process of step 2).
Signed-off-by: Chao Bi <chao.bi@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>