bcachefs: Fix version checks in validate_bset()

It seems btree node scan picked up a partially overwritten btree node,
and corrected the "bset version older than sb version_min" error -
resulting in an invalid superblock with a bad version_min field.

Don't run this check at all when we're in btree node scan, and when we
do run it, do something saner if the bset version is totally crazy.

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