NFS: Introduce NFS_ATTR_FATTR_V4_LOCATIONS

The Linux NFS client must distinguish between referral events (which
it currently supports) and migration events (which it does not yet
support).

In both types of events, an fs_locations array is returned.  But upper
layers, not the XDR layer, should make the distinction between a
referral and a migration.  There really isn't a way for an XDR decoder
function to distinguish the two, in general.

Slightly adjust the FATTR flags returned by decode_fs_locations()
to set NFS_ATTR_FATTR_V4_LOCATIONS only if a non-empty locations
array was returned from the server.  Then have logic in nfs4proc.c
distinguish whether the locations array is for a referral or
something else.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>