axmap: fix deadlock

axmap: fix deadlock

axmap_first_free() is always called with axmap->lock held,
so needn't to acquire the lock inside the function.

The deadlock is introduced in commit 12bde3697fc230d7a(axmap:
ensure we lock down the maps for shared access).

Given axmap_first_free() is only called inside lib/axmap.c,
this patch declares the function as static. In the future,
if external users need the function, axmap lock can be
considered at that time.

Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>