bcachefs: Simplify hash table checks

Very early on there was a period where we were accidentally generating
dirents with trailing garbage; we've since dropped support for
filesystems that old and the fsck code can be dropped.

Also, this patch switches to a simpler algorithm for checking hash
tables. It's less efficient on hash collision - but with 64 bit keys,
those are very rare.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>