nfsd: wire up GET_DIR_DELEGATION handling

nfsd: wire up GET_DIR_DELEGATION handling

Add a new routine for acquiring a read delegation on a directory. These
are recallable-only delegations with no support for CB_NOTIFY. That will
be added in a later phase.

Since the same CB_RECALL/DELEGRETURN infrastructure is used for regular
and directory delegations, a normal nfs4_delegation is used to represent
a directory delegation.

Reviewed-by: NeilBrown <neil@brown.name>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20251111-dir-deleg-ro-v6-16-52f3feebb2f2@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>