bcachefs: Go rw before journal replay

This gets us slightly nicer log messages.

Also, this slightly clarifies synchronization of c->journal_keys; after
we go RW it's in use by multiple threads (so that the btree iterator
code can overlay keys from the journal); so it has to be prepped before
that point.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>