filelock: lift the ban on directory leases in generic_setlease

filelock: lift the ban on directory leases in generic_setlease

With the addition of the try_break_lease calls in directory changing
operations, allow generic_setlease to hand them out. Write leases on
directories are never allowed however, so continue to reject them.

For now, there is no API for requesting delegations from userland, so
ensure that userland is prevented from acquiring a lease on a directory.

Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20251111-dir-deleg-ro-v6-13-52f3feebb2f2@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>