nsfs: add compat ioctl handler

As all parameters and return values of the ioctls have the same
representation on both 32bit and 64bit we can reuse the normal ioctl
handler for the compat handler via compat_ptr_ioctl().

All nsfs ioctls return a plain "int" filedescriptor which is a signed
4-byte integer type on both 32bit and 64bit.
The only parameter taken is by NS_GET_OWNER_UID and is a pointer to a
"uid_t" which is a 4-byte unsigned integer type on both 32bit and 64bit.

Fixes: 6786741 ("nsfs: add ioctl to get an owning user namespace for ns file descriptor")
Reported-by: Karel Zak <kzak@redhat.com>
Link: util-linux/util-linux#1924 (comment)
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/nsfs.c

@@ -21,6 +21,7 @@ static long ns_ioctl(struct file *filp, unsigned int ioctl,
 static const struct file_operations ns_file_operations = {
 	.llseek		= no_llseek,
 	.unlocked_ioctl = ns_ioctl,
+	.compat_ioctl   = compat_ptr_ioctl,
 };
 
 static char *ns_dname(struct dentry *dentry, char *buffer, int buflen)