bcachefs: Kill __bch2_accounting_mem_mod()

The next patch will be adding a disk accounting counter type which is
not kept in the in-memory eytzinger tree.

As prep, fold __bch2_accounting_mem_mod() into
bch2_accounting_mem_mod_locked() so that we can check for that counter
type and bail out without calling bpos_to_disk_accounting_pos() twice.

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