storage/posix: Remove special handling of ENOTSUP for [f]setxattr.

Change-Id: I7df7b2263336af0abe5bc91c674f9401aff6c3e0
BUG: 1098794
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/7788
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>