NFSD: Do the grace period check in ->proc_layoutget

RFC 8881 Section 18.43.3 states:
> If the metadata server is in a grace period, and does not persist
> layouts and device ID to device address mappings, then it MUST
> return NFS4ERR_GRACE (see Section 8.4.2.1).

Jeff observed that this suggests the grace period check is better
done by the individual layout type implementations, because checking
for the server grace period is unnecessary for some layout types.

Suggested-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/linux-nfs/7h5p5ktyptyt37u6jhpbjfd5u6tg44lriqkdc7iz7czeeabrvo@ijgxz27dw4sg/T/#t


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