Fix a little of the coding style in mm/mmap.c

[akpm@linux-foundation.org: cleanup]
Signed-off-by: NZhenwenXu <helight.xu@gmail.com>
Signed-off-by: NHugh Dickins <hugh@veritas.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>