fuse: fix clearing suid, sgid for chown()

Basically, the pjdfstests set the ownership of a file to 06555, and then
chowns it (as root) to a new uid/gid. Prior to commit a09f99eddef4 ("fuse:
fix killing s[ug]id in setattr"), fuse would send down a setattr with both
the uid/gid change and a new mode.  Now, it just sends down the uid/gid
change.

Technically this is NOTABUG, since POSIX doesn't _require_ that we clear
these bits for a privileged process, but Linux (wisely) has done that and I
think we don't want to change that behavior here.

This is caused by the use of should_remove_suid(), which will always return
0 when the process has CAP_FSETID.

In fact we really don't need to be calling should_remove_suid() at all,
since we've already been indicated that we should remove the suid, we just
don't want to use a (very) stale mode for that.

This patch should fix the above as well as simplify the logic.

Reported-by: Jeff Layton <jlayton@redhat.com> 
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: a09f99eddef4 ("fuse: fix killing s[ug]id in setattr")
Cc: <stable@vger.kernel.org>
Reviewed-by: Jeff Layton <jlayton@redhat.com>

fs/fuse/dir.c

@@ -1739,8 +1739,6 @@ static int fuse_setattr(struct dentry *entry, struct iattr *attr)
 		 * This should be done on write(), truncate() and chown().
 		 */
 		if (!fc->handle_killpriv) {
-			int kill;
-
 			/*
 			 * ia_mode calculation may have used stale i_mode.
 			 * Refresh and recalculate.
@@ -1750,12 +1748,11 @@ static int fuse_setattr(struct dentry *entry, struct iattr *attr)
 				return ret;
 
 			attr->ia_mode = inode->i_mode;
-			kill = should_remove_suid(entry);
-			if (kill & ATTR_KILL_SUID) {
+			if (inode->i_mode & S_ISUID) {
 				attr->ia_valid |= ATTR_MODE;
 				attr->ia_mode &= ~S_ISUID;
 			}
-			if (kill & ATTR_KILL_SGID) {
+			if ((inode->i_mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
 				attr->ia_valid |= ATTR_MODE;
 				attr->ia_mode &= ~S_ISGID;
 			}