bcachefs: Fix marking reflink pointers to missing indirect extents

reflink pointers to missing indirect extents aren't deleted, they just
have an error bit set - in case the indirect extent somehow reappears.

fsck/mark and sweep thus needs to ignore these errors.

Also, they can be marked AUTOFIX now.

Reported-by: Roland Vet <vet.roland@protonmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>