make fb_deferred_io_mkwrite() static

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Cc: Jaya Kumar <jayakumar.lkml@gmail.com>
Cc: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>