porting: Provide fallocate and fremovexattr for OSX

Change-Id: I563216f83edaff6d01a251ef0c1746a14aec700c
BUG: 1089172
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/8217
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>