NFS: Don't silently fail setattr() requests on mountpoints

NFS: Don't silently fail setattr() requests on mountpoints

commit ab225417825963b6dc66be7ea80f94ac1378dfdf upstream.

Ensure that any setattr and getattr requests for junctions and/or
mountpoints are sent to the server. Ever since commit
0ec26fd0698 (vfs: automount should ignore LOOKUP_FOLLOW), we have
silently dropped any setattr requests to a server-side mountpoint.
For referrals, we have silently dropped both getattr and setattr
requests.

This patch restores the original behaviour for setattr on mountpoints,
and tries to do the same for referrals, provided that we have a
filehandle...

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>