fbdev: add mutex for fb_mmap locking

fbdev: add mutex for fb_mmap locking

Add a mutex to avoid a circular locking problem between the mm layer
semaphore and fbdev ioctl mutex through the fb_mmap() call.

Also, add mutex to all places where smem_start and smem_len fields change
so the mutex inside the fb_mmap() is actually used.  Changing of these
fields before calling the framebuffer_register() are not mutexed.

This is 2.6.31 material.  It removes one lockdep (fb_mmap() and
register_framebuffer()) but there is still another one (fb_release() and
register_framebuffer()).  It also cleans up handling of the smem_start and
smem_len fields used by mutexed section of the fb_mmap().

Signed-off-by: Krzysztof Helt <redacted>
Cc: Peter Zijlstra <redacted>
Cc: "Rafael J. Wysocki" <redacted>
Cc: <redacted>
Signed-off-by: Andrew Morton <redacted>
Signed-off-by: Linus Torvalds <redacted>