filelocks: use mount idmapping for setlease permission check

A user should be allowed to take out a lease via an idmapped mount if
the fsuid matches the mapped uid of the inode. generic_setlease() is
checking the unmapped inode uid, causing these operations to be denied.

Fix this by comparing against the mapped inode uid instead of the
unmapped uid.

Fixes: 9caccd41 ("fs: introduce MOUNT_ATTR_IDMAP")
Cc: stable@vger.kernel.org
Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>

fs/locks.c

@@ -1863,9 +1863,10 @@ int generic_setlease(struct file *filp, long arg, struct file_lock **flp,
 			void **priv)
 {
 	struct inode *inode = file_inode(filp);
+	vfsuid_t vfsuid = i_uid_into_vfsuid(file_mnt_idmap(filp), inode);
 	int error;
 
-	if ((!uid_eq(current_fsuid(), inode->i_uid)) && !capable(CAP_LEASE))
+	if ((!vfsuid_eq_kuid(vfsuid, current_fsuid())) && !capable(CAP_LEASE))
 		return -EACCES;
 	if (!S_ISREG(inode->i_mode))
 		return -EINVAL;