posix: leverage FALLOC_FL_ZERO_RANGE in zerofill fop

posix_zerofill() implements zerofilling of a given (offset,length) by
doing a writev in a loop followed by an optional fsync on the file.
fallocate(2) has a FALLOC_FL_ZERO_RANGE flag which does away with all
this and provides the same result (from a userspace application point of
view) with a single syscall. This patch attempts the zerofill with the
latter and falls back to the former if it fails.

Tested using a libgfapi based C program on XFS and observed using gdb that
posix_zerofill()'s call to fallocate with FALLOC_FL_ZERO_RANGE was a
success.

Change-Id: I77e9b7de0d59c255f06b0c39c43a276990081727
BUG: 1361249
Signed-off-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-on: http://review.gluster.org/15037
Smoke: Gluster Build System <jenkins@build.gluster.org>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
NetBSD-regression: NetBSD Build System <jenkins@build.gluster.org>
CentOS-regression: Gluster Build System <jenkins@build.gluster.org>

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