Revert "UBUNTU: SAUCE: (namespace) fs: Don't remove suid for CAP_FSETID for userns root"

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

This reverts commit bd088dae.

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/inode.c

@@ -1677,8 +1677,7 @@ EXPORT_SYMBOL(touch_atime);
  */
 int should_remove_suid(struct dentry *dentry)
 {
-	struct inode *inode = d_inode(dentry);
-	umode_t mode = inode->i_mode;
+	umode_t mode = d_inode(dentry)->i_mode;
 	int kill = 0;
 
 	/* suid always must be killed */
@@ -1692,8 +1691,7 @@ int should_remove_suid(struct dentry *dentry)
 	if (unlikely((mode & S_ISGID) && (mode & S_IXGRP)))
 		kill |= ATTR_KILL_SGID;
 
-	if (unlikely(kill && !capable_wrt_inode_uidgid(inode, CAP_FSETID) &&
-		     S_ISREG(mode)))
+	if (unlikely(kill && !capable(CAP_FSETID) && S_ISREG(mode)))
 		return kill;
 
 	return 0;