namei: ignore ERR/NULL names in putname()

Supporting ERR/NULL names in putname() makes callers code cleaner, and
is what some other path walking functions already support for the same
reason.

This also removes a few existing IS_ERR checks before putname().
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/io-uring/CAHk-=wgCac9hBsYzKMpHk0EbLgQaXR=OUAjHaBtaY+G8A9KhFg@mail.gmail.com/Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Dmitry Kadashev <dkadashev@gmail.com>
Link: https://lore.kernel.org/r/20210708063447.3556403-2-dkadashev@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/namei.c

@@ -247,6 +247,9 @@ getname_kernel(const char * filename)
 
 void putname(struct filename *name)
 {
+	if (IS_ERR_OR_NULL(name))
+		return;
+
 	BUG_ON(name->refcnt <= 0);
 
 	if (--name->refcnt > 0)
@@ -4728,11 +4731,9 @@ int do_renameat2(int olddfd, struct filename *from, int newdfd,
 		goto retry;
 	}
 put_both:
-	if (!IS_ERR(from))
-		putname(from);
+	putname(from);
 put_new:
-	if (!IS_ERR(to))
-		putname(to);
+	putname(to);
 	return error;
 }