object-storage: Use fchown instead of chown.

This is a step towards making fd based system calls where ever possible to avoid
path lookups.

Signed-off-by: Mohammed Junaid <junaid@redhat.com>
Change-Id: I482ea29ebe0859d0a5307ff25ecb5945d54bc7ca
Reviewed-on: http://review.gluster.org/5251
Reviewed-by: Peter Portante <pportant@redhat.com>
Tested-by: Peter Portante <pportant@redhat.com>