bcachefs: Journal resize fixes

- Fix a sleeping-in-atomic bug due to calling
bch2_journal_buckets_to_sb() under the journal lock.
- Additionally, now we mark buckets as journal buckets before adding
them to the journal in memory and the superblock. This ensures that
if we crash part way through we'll never be writing to journal
buckets that aren't marked correctly.

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

bcachefs: bch_sb_field_journal_v2

Add a new superblock field which represents journal buckets as ranges:
also move code for the superblock journal fields to journal_sb.c.

This also reworks the code for resizing the journal to write the new
superblock before using the new journal buckets, and thus be a bit
safer.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>