Revert "(namespace) fs: Refuse uid/gid changes which don't map into s_user_ns"

BugLink: https://bugs.launchpad.net/bugs/1644165

This reverts commit dc0d66cb.

The kernel fix for bug #1634964 breaks LXD userspace, in particular the
following commits:

ac7f3f73 (namespace) vfs: Don't modify inodes with a uid or gid unknown to the vfs
ca52383a (namespace) vfs: Don't create inodes with a uid or gid unknown to the vfs

LXD 2.0.6 will include changes to support these kernel changes, but it isn't
available yet on xenial, so for now we just revert these commits.
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

fs/attr.c

@@ -301,17 +301,6 @@ int notify_change(struct dentry * dentry, struct iattr * attr, struct inode **de
 	if (!(attr->ia_valid & ~(ATTR_KILL_SUID | ATTR_KILL_SGID)))
 		return 0;
 
-	/*
-	 * Verify that uid/gid changes are valid in the target
-	 * namespace of the superblock.
-	 */
-	if (ia_valid & ATTR_UID &&
-	    !kuid_has_mapping(inode->i_sb->s_user_ns, attr->ia_uid))
-		return -EOVERFLOW;
-	if (ia_valid & ATTR_GID &&
-	    !kgid_has_mapping(inode->i_sb->s_user_ns, attr->ia_gid))
-		return -EOVERFLOW;
-
 	error = security_inode_setattr(dentry, attr);
 	if (error)
 		return error;