mm/mmap.c: optimization of do_mmap_pgoff function

The simple check for zero length memory mapping may be performed
earlier.  So that in case of zero length memory mapping some unnecessary
code is not executed at all.  It does not make the code less readable
and saves some CPU cycles.

Signed-off-by: Piotr Kwapulinski <kwapulinski.piotr@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>