changelog: correct mmap() error checking

Upon failure, mmap() returns MAP_FAILED, which is not equal to NULL.
This small correction makes sure that the potential error gets caught
and handled.

BUG: 1138621
Change-Id: I0a88b5187afa7187dcaa8f7d2cb0f9bb775c929d
Reported-by: Santosh Pradhan <santosh.pradhan@gmail.com>
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/9130
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Venky Shankar <vshankar@redhat.com>
Tested-by: Venky Shankar <vshankar@redhat.com>