bcachefs: Small check_fix_ptr fixes

We don't want to change the bucket gen, on gen mismatch: it's possible
to have multiple btree nodes with different gens in the same bucket that
we want to keep, if we have to recover from btree node scan.

It's also not necessary to set g->gen_valid; add a comment to that
effect.

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