Merge patch series "fscontext: do not consume log entries when returning -EMSGSIZE"

Aleksa Sarai <cyphar@cyphar.com> says:

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.

* patches from https://lore.kernel.org/20250807-fscontext-log-cleanups-v3-0-8d91d6242dc3@cyphar.com:
  selftests/filesystems: add basic fscontext log tests
  fscontext: do not consume log entries when returning -EMSGSIZE

Link: https://lore.kernel.org/20250807-fscontext-log-cleanups-v3-0-8d91d6242dc3@cyphar.com
Signed-off-by: Christian Brauner <brauner@kernel.org>