btrfs: rip out may_commit_transaction

may_commit_transaction was introduced before the ticketing
infrastructure existed.  There was a problem where we'd legitimately be
out of space, but every reservation would trigger a transaction commit
and then fail.  Thus if you had 1000 things trying to make a
reservation, they'd all do the flushing loop and thus commit the
transaction 1000 times before they'd get their ENOSPC.

This helper was introduced to short circuit this, if there wasn't space
that could be reclaimed by committing the transaction then simply ENOSPC
out.  This made true ENOSPC tests much faster as we didn't waste a bunch
of time.

However many of our bugs over the years have been from cases where we
didn't account for some space that would be reclaimed by committing a
transaction.  The delayed refs rsv space, delayed rsv, many pinned bytes
miscalculations, etc.  And in the meantime the original problem has been
solved with ticketing.  We no longer will commit the transaction 1000
times.  Instead we'll get 1000 waiters, we will go through the flushing
mechanisms, and if there's no progress after 2 loops we ENOSPC everybody
out.  The ticketing infrastructure gives us a deterministic way to see
if we're making progress or not, thus we avoid a lot of extra work.

So simplify this step by simply unconditionally committing the
transaction.  This removes what is arguably our most common source of
early ENOSPC bugs and will allow us to drastically simplify many of the
things we track because we simply won't need them with this stuff gone.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>