mm/mmap.c: remove redundant statement "error = -ENOMEM"

It is still a little better to remove it, although it should be skipped
by "-O2".

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com&gt;=0A=>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>