bcachefs: KEY_TYPE_error now counts towards i_sectors

KEY_TYPE_error is used when all replicas in an extent are marked as
failed; it indicates that data was present, but has been lost.

So that i_sectors doesn't change when replacing extents with
KEY_TYPE_error, we now have to count error keys as allocations - this
fixes fsck errors later.

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