Do not forbid fallocate on non Linux systems

Linux fallocate() differs from posix_fallocate() by
an extra flag that can have the FALLOC_FL_KEEP_SIZE value;

Do not test FALLOC_FL_KEEP_SIZE existence to enable fallocate()
in posix xlator, as sys_fallocate() in libglusterfs provides
support for both implementations.

Backport of Idf41a0396028a15e81281791bf6912d7fd674e3f
BUG: 1138897
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>

Change-Id: Ie6e5ea923561630c52a6db5c7f83313cfdc34811
Reviewed-on: http://review.gluster.org/8862
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>