NFS: Fix a potential NULL dereference in nfs_get_client()

None of the callers are expecting NULL returns from nfs_get_client() so
this code will lead to an Oops.  It's better to return an error
pointer.  I expect that this is dead code so hopefully no one is
affected.

Fixes: 31434f49

 ("nfs: check hostname in nfs_get_client")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>