In rare cases, bch2_check_extents_to_backpointers() would incorrectly
flag an extent has having a missing backpointer when we just needed to
flush the btree write buffer - we weren't tracking the last flushed
position correctly.

This adds a level field to the last_flushed pos, fixing a bug where we'd
sometimes fail on a new root node.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>