nfsserver: Error-check unbind_tree

[nrwahl2]

Fail to stop if unmounting rpcpipefs_dir or /var/lib/nfs fails.

After:

# pcs resource debug-stop nfs-daemon
Operation stop for nfs-daemon (ocf:heartbeat:nfsserver) returned: 'error' (1)
Feb 02 17:58:58 INFO: Stopping NFS server ...
Feb 02 17:58:58 INFO: Stop: threads
Feb 02 17:58:58 INFO: Stop: rpc-statd
Feb 02 17:58:58 INFO: Stop: nfs-idmapd
Feb 02 17:58:58 INFO: Stop: nfs-mountd
Feb 02 17:58:58 INFO: Stop: nfsdcld
Feb 02 17:58:58 INFO: Stop: rpc-gssd
Feb 02 17:58:58 INFO: Stop: umount (1/10 attempts)
umount: /var/lib/nfs: target is busy.
Feb 02 17:58:59 ERROR: Failed to unmount /var/lib/nfs
ocf-exit-reason:Failed to unmount a bind mount

Resolves: RHBZ#1924363

Signed-off-by: Reid Wahl nrwahl@protonmail.com

[oalbrigt]

LGTM.