gluster: Fix use after free in glfs_clear_preopened()

This fixes a use-after-free bug introduced in commit 6349c154. We need
to use QLIST_FOREACH_SAFE() when freeing elements in the loop. Spotted
by Coverity.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 1479378608-11962-1-git-send-email-kwolf@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>
