net: msg_zerocopy: elide page accounting if RLIM_INFINITY

MSG_ZEROCOPY ensures that pinned user pages do not exceed the limit.
If no limit is set, skip this accounting as otherwise expensive
atomic_long operations are called for no reason.

This accounting is already skipped for privileged (CAP_IPC_LOCK)
users. Rely on the same mechanism: if no mmp->user is set,
mm_unaccount_pinned_pages does not decrement either.

Tested by running tools/testing/selftests/net/msg_zerocopy.sh with
an unprivileged user for the TXMODE binary:

    ip netns exec "${NS1}" sudo -u "{$USER}" "${BIN}" "-${IP}" ...
Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20230214155740.3448763-1-willemdebruijn.kernel@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/skbuff.c

@@ -1406,14 +1406,18 @@ EXPORT_SYMBOL_GPL(skb_morph);
 
 int mm_account_pinned_pages(struct mmpin *mmp, size_t size)
 {
-	unsigned long max_pg, num_pg, new_pg, old_pg;
+	unsigned long max_pg, num_pg, new_pg, old_pg, rlim;
 	struct user_struct *user;
 
 	if (capable(CAP_IPC_LOCK) || !size)
 		return 0;
 
+	rlim = rlimit(RLIMIT_MEMLOCK);
+	if (rlim == RLIM_INFINITY)
+		return 0;
+
 	num_pg = (size >> PAGE_SHIFT) + 2;	/* worst case */
-	max_pg = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
+	max_pg = rlim >> PAGE_SHIFT;
 	user = mmp->user ? : current_user();
 
 	old_pg = atomic_long_read(&user->locked_vm);