fs: Don't remove suid for CAP_FSETID in s_user_ns

Expand the check in should_remove_suid() to keep privileges for
CAP_FSETID in s_user_ns rather than init_user_ns.
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>

fs/inode.c

@@ -1683,7 +1683,8 @@ int should_remove_suid(struct dentry *dentry)
 	if (unlikely((mode & S_ISGID) && (mode & S_IXGRP)))
 		kill |= ATTR_KILL_SGID;
 
-	if (unlikely(kill && !capable(CAP_FSETID) && S_ISREG(mode)))
+	if (unlikely(kill && !ns_capable(dentry->d_sb->s_user_ns, CAP_FSETID) &&
+		     S_ISREG(mode)))
 		return kill;
 
 	return 0;