bcachefs: restart journal reclaim thread on ro->rw transitions

bcachefs: restart journal reclaim thread on ro->rw transitions

Commit c2d5ff36065a4 ("bcachefs: Start journal reclaim thread
earlier") tweaked reclaim thread management to start a bit earlier
in the mount sequence by moving the start call from
__bch2_fs_read_write() to bch2_fs_journal_start(). This has the side
effect of never starting the reclaim thread on a ro->rw transition,
which can be observed by monitoring reclaim behavior via the
journal_reclaim tracepoints. I.e. once an fs has remounted ro->rw,
we only ever rely on direct reclaim from that point forward.

Since bch2_journal_reclaim_start() properly handles the case where
the reclaim thread has already been created, restore the start call
in the read-write helper. This allows the reclaim thread to start
early when appropriate and also exit/restart on remounts or freeze
cycles. In the latter case it may be possible to simply allow the
task to freeze rather than destroy it, but for now just fix the
immediate bug.

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