bcachefs: Use for_each_btree_key_upto() more consistently

bcachefs: Use for_each_btree_key_upto() more consistently

It's important that in BTREE_ITER_FILTER_SNAPSHOTS mode we always use
peek_upto() and provide an end for the interval we're searching for -
otherwise, when we hit the end of the inode the next inode be in a
different subvolume and not have any keys in the current snapshot, and
we'd iterate over arbitrarily many keys before returning one.

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