nfsd: handle vfs_getattr errors in acl protocol

We're currently ignoring errors from vfs_getattr.

The correct thing to do is to do the stat in the main service procedure
not in the response encoding.

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>