nfsd: Do not let nfs4_file pin the struct inode

Remove the fi_inode field in struct nfs4_file in order to remove the
possibility of struct nfs4_file pinning the inode when it does not have
any open state.

The only place we still need to get to an inode is in check_for_locks,
so change it to use find_any_file and use the inode from any that it
finds. If it doesn't find one, then just assume there aren't any.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>