Two per-CPU variables are allocated as pointer to per-CPU memory which
then are used as scratch buffers.
We could be smart about this and use instead a per-CPU struct which
contains the pointers already and then we need to allocate just the
scratch buffers.
Add a lock to the struct. By doing so we can avoid the get_cpu()
statement and gain lockdep coverage (if enabled) to ensure that the lock
is always acquired in the right context. On non-preemptible kernels the
lock vanishes.
It is okay to use raw_cpu_ptr() in order to get a pointer to the struct
since it is protected by the spinlock.

The diffstat of this is negative and according to size scompress.o:
   text    data     bss     dec     hex filename
   1847     160      24    2031     7ef dbg_before.o
   1754     232       4    1990     7c6 dbg_after.o
   1799      64      24    1887     75f no_dbg-before.o
   1703      88       4    1795     703 no_dbg-after.o

The overall size increase difference is also negative. The increase in
the data section is only four bytes without lockdep.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>