bcachefs: Ensure BCH_FS_may_go_rw is set before exiting recovery

If BCH_FS_may_go_rw is not yet set, it indicates to the transaction
commit path that updates should be done via the list of journal replay
keys.

This must be set before multithreaded use commences.

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