bcachefs: check_extent(): don't use key_visible_in_snapshot()

bcachefs: check_extent(): don't use key_visible_in_snapshot()

This changes the main part of check_extents(), that checks the extent
against the corresponding inode, to not use key_visible_in_snapshot().

key_visible_in_snapshot() has to iterate over the list of ancestor
overwrites repeatedly calling bch2_snapshot_is_ancestor(), so this is a
significant performance improvement.

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