Previously, btree_update_interior.c passed keys to bch2_trans_mark_*
that hadn't been fully initialized - they didn't have the key field
filled out, just the value.

With backpointers, we need to make sure keys are fully initialized
before marking them - because the backpointer points back to the
original key.

This patch tweaks the interior update paths to fix this.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>