bcachefs: Improve journal entry validate code

bcachefs: Improve journal entry validate code

Previously, the journal entry read code was changed so that if we got a
journal entry that failed validation, we'd try to use it, preferring to
use a good version from another device if available.

But this left a bug where if an earlier validation check (say, checksum)
failed, the later checks (for last_seq) wouldn't run and we'd end up
using a journal entry with a garbage last_seq field. This fixes that so
that the later validation checks run and if necessary change those
fields to something sensible.

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