bcachefs: Extensive triggers cleanups

bcachefs: Extensive triggers cleanups

 - We no longer mark subsets of extents, they're marked like regular
   keys now - which means we can drop the offset & sectors arguments
   to trigger functions
 - Drop other arguments that are no longer needed anymore in various
   places - fs_usage
 - Drop the logic for handling extents in bch2_mark_update() that isn't
   needed anymore, to match bch2_trans_mark_update()
 - Better logic for hanlding the BTREE_ITER_CACHED_NOFILL case, where we
   don't have an old key to mark

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