rbd_wait_state_locked() is built around rbd_dev->lock_waitq and blocks
rbd worker threads while waiting for the lock, potentially impacting
other rbd devices.  There is no good way to pass an error code into
image request state machines when acquisition fails, hence the use of
RBD_DEV_FLAG_BLACKLISTED for everything and various other issues.

Introduce rbd_dev->acquiring_list and move acquisition into image
request state machine.  Use rbd_img_schedule() for kicking and passing
error codes.  No blocking occurs while waiting for the lock, but
rbd_dev->lock_rwsem is still held across lock, unlock and set_cookie
calls.

Always acquire the lock on "rbd map" to avoid associating the latency
of acquiring the lock with the first I/O request.

A slight regression is that lock_timeout is now respected only if lock
acquisition is triggered by "rbd map" and not by I/O.  This is somewhat
compensated by the fact that we no longer block if the peer refuses to
release lock -- I/O is failed with EROFS right away.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Dongsheng Yang <dongsheng.yang@easystack.cn>