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.

BUG: 1129939
Change-Id: Idf41a0396028a15e81281791bf6912d7fd674e3f
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8856
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>