- X-macro-ize the bch_folio_sector_state enum: this means we can easily
   generate strings, which is helpful for debugging.

 - Add helpers for state transitions: folio_sector_dirty(),
   folio_sector_undirty(), folio_sector_reserve()

 - Add folio_sector_set(), a single helper for changing folio sector
   state just so that we have a single place to instrument when we're
   debugging.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>