bcachefs: Call bch2_fs_init_rw() early if we'll be going rw

bcachefs: Call bch2_fs_init_rw() early if we'll be going rw

kthread creation checks for pending signals, which is _very_ annoying if
we have to do a long recovery and don't go rw until we've done
significant work.

Check if we'll be going rw and pre-allocate kthreads/workqueues.

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