bcachefs: Put bkey invalid check in commit path in a more useful place

bcachefs: Put bkey invalid check in commit path in a more useful place

When doing updates early in recovery, before we can go RW, we still want
to check that keys are valid at commit time - this moves key invalid
checking to before the "btree updates to journal" path.

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