nfs: properly disallow delegation requests on directories

nfs: properly disallow delegation requests on directories

Checking for S_ISREG() in nfs4_setlease() is incorrect, since that op is
never called for directories. The right way to deny lease requests on
directories is to set the ->setlease() operation to simple_nosetlease()
in the directory file_operations.

Fixes: e6d28ebc17eb ("filelock: push the S_ISREG check down to ->setlease handlers")
Reported-by: Christoph Hellwig <hch@infradead.org>
Closes: https://lore.kernel.org/linux-fsdevel/aV316LhsVSl0n9-E@infradead.org/
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260107-setlease-6-19-v1-1-85f034abcc57@kernel.org
Tested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>