fbmem: fix fb_info->lock and mm->mmap_sem circular locking dependency

Fix a circular locking dependency in the frame buffer console driver
pushing down the mutex fb_info->lock.

Circular locking dependecies occur calling the blocking
fb_notifier_call_chain() with fb_info->lock held.  Notifier callbacks can
try to acquire mm->mmap_sem, while fb_mmap() acquires the locks in the
reverse order mm->mmap_sem => fb_info->lock.

Tested-by: Andrey Borzenkov <arvidjaar@mail.ru>
Signed-off-by: Andrea Righi <righi.andrea@gmail.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>