fscontext: do not consume log entries when returning -EMSGSIZE

fscontext: do not consume log entries when returning -EMSGSIZE

commit 72d271a7baa7062cb27e774ac37c5459c6d20e22 upstream.

Userspace generally expects APIs that return -EMSGSIZE to allow for them
to adjust their buffer size and retry the operation. However, the
fscontext log would previously clear the message even in the -EMSGSIZE
case.

Given that it is very cheap for us to check whether the buffer is too
small before we remove the message from the ring buffer, let's just do
that instead. While we're at it, refactor some fscontext_read() into a
separate helper to make the ring buffer logic a bit easier to read.

Fixes: 007ec26cdc9f ("vfs: Implement logging through fs_context")
Cc: David Howells <dhowells@redhat.com>
Cc: stable@vger.kernel.org # v5.2+
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Link: https://lore.kernel.org/20250807-fscontext-log-cleanups-v3-1-8d91d6242dc3@cyphar.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>