Blackfin arch: after removing fs.h from mm.h, fix the broken on Blackfin arch

Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>