bcachefs: Don't delete reflink pointers to missing indirect extents

bcachefs: Don't delete reflink pointers to missing indirect extents

To avoid tragic loss in the event of transient errors (i.e., a btree
node topology error that was later corrected by btree node scan), we
can't delete reflink pointers to correct errors.

This adds a new error bit to bch_reflink_p, indicating that it is known
to point to a missing indirect extent, and the error has already been
reported.

Indirect extent lookups now use bch2_lookup_indirect_extent(), which on
error reports it as a fsck_err() and sets the error bit, and clears it
if necessary on succesful lookup.

This also gets rid of the bch2_inconsistent_error() call in
__bch2_read_indirect_extent, and in the reflink_p trigger: part of the
online self healing project.

An on disk format change isn't necessary here: setting the error bit
will be interpreted by older versions as pointing to a different index,
which will also be missing - which is fine.

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