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 <[e-mail]>
Closes: https://lore.kernel.org/linux-fsdevel/[e-mail]/
Signed-off-by: Jeff Layton <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Tested-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

fs/nfs/dir.c

···
 	.open		= nfs_opendir,
 	.release	= nfs_closedir,
 	.fsync		= nfs_fsync_dir,
+	.setlease	= simple_nosetlease,
 };
 
 const struct address_space_operations nfs_dir_aops = {

fs/nfs/nfs4file.c

···
 static int nfs4_setlease(struct file *file, int arg, struct file_lease **lease,
 			 void **priv)
 {
-	if (!S_ISREG(file_inode(file)->i_mode))
-		return -EINVAL;
 	return nfs4_proc_setlease(file, arg, lease, priv);
 }