NFS: Fix a double unlock from nfs_match,get_client

Now that nfs_match_client drops the nfs_client_lock, we should be
careful
to always return it in the same condition: locked.

Fixes: 950a578c ("NFS: make nfs_match_client killable")
Reported-by:  <syzbot+228a82b263b5da91883d@syzkaller.appspotmail.com>
Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>