nfs: if we have no valid attrs, then don't declare the attribute cache valid

nfs: if we have no valid attrs, then don't declare the attribute cache valid

commit c812012f9ca7cf89c9e1a1cd512e6c3b5be04b85 upstream.

If we pass in an empty nfs_fattr struct to nfs_update_inode, it will
(correctly) not update any of the attributes, but it then clears the
NFS_INO_INVALID_ATTR flag, which indicates that the attributes are
up to date. Don't clear the flag if the fattr struct has no valid
attrs to apply.

Reviewed-by: Steve French <steve.french@primarydata.com>
Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
(cherry picked from commit ddab0155aa9589600861e3f65d505982b719496a)
Signed-off-by: Willy Tarreau <w@1wt.eu>