bcachefs: drop unnecessary journal stuck check from space calculation

bcachefs: drop unnecessary journal stuck check from space calculation

The journal stucking check in bch2_journal_space_available() is
particularly aggressive and can lead to premature shutdown in some
rare cases. This is difficult to reproduce, but also comes along
with a fatal error and so is worthwhile to be cautious.

For example, we've seen instances where the journal is under heavy
reservation pressure, the journal allocation path transitions into
the final available journal bucket, the journal write path
immediately consumes that bucket and calls into
bch2_journal_space_available(), which then in turn flags the journal
as stuck because there is no available space and shuts down the
filesystem instead of submitting the journal write (that would have
otherwise succeeded).

To avoid this problem, simplify the journal stuck checking by just
relying on the higher level logic in the journal reservation path.
This produces more useful debug output and is a more reliable
indicator that things have bogged down.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>