bcachefs: Run check_extents_to_backpointers() in multiple passes

bcachefs: Run check_extents_to_backpointers() in multiple passes

Similer to the previous patch for check_backpointers_to_extents(), if
the alloc + backpointers btrees do not fit in ram we need to run into
multiple passes.

The counting of btree nodes that fit in memory is different here,
because we have to walk the alloc and backpointers btrees at the same
time, since a backpointer could reside in either of them and we don't
know which without checking both.

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