MEDIUM: ring: attach readers under the tail lock instead of isolating

MEDIUM: ring: attach readers under the tail lock instead of isolating

ring_dispatch_messages() isolates the thread to grab the initial reader
offset, as introduced by commit 31b93b40b ("MEDIUM: ring: protect the
initialization of the initial reader offset"), whose message already
noted that the tail lock could be used instead.

Doing it that way removes a trap for the callers: isolating from a
function that may be called with a lock held deadlocks against any
thread spinning on that lock, which is what happened with the sink
forward applet. Only a writer holding the tail lock may move the head,
so taking that same lock is sufficient to perform the initialization
while we attach.

Contrary to the isolation, this does not exclude a writer that released
the tail lock and is still copying its payload, so the readers count
byte may transiently be seen as RING_WRITING_SIZE if we're unlucky,
but the existing retry loop on values above RING_MAX_READERS already
handles that case.

This also makes attaching a reader a lot cheaper even if that has
never been a problem.

Note that this only aims at making the code more robust and tolerant
to accidental abuse, it's not a reason for calling
ring_dispatch_messages() under a lock anyway!