bcachefs: Fix check_backpointers_to_extents range limiting

bcachefs: Fix check_backpointers_to_extents range limiting

bch2_get_btree_in_memory_pos() will return positions that refer directly
to the btree it's checking will fit in memory - i.e. backpointer
positions, not buckets.

This also means check_bp_exists() no longer has to refer to the device,
and we can delete some code.

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