bcachefs: Fix handling of unknown bkey types

min_val_size was U8_MAX for unknown key types, causing us to flag any
known key as invalid - it should have been 0.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bkey_methods.c

@@ -121,7 +121,6 @@ const struct bkey_ops bch2_bkey_ops[] = {
 };
 
 const struct bkey_ops bch2_bkey_null_ops = {
-	.min_val_size = U8_MAX,
 };
 
 int bch2_bkey_val_invalid(struct bch_fs *c, struct bkey_s_c k,