bcachefs: Switch to unsafe_memcpy() in a few places

The new fortify checking doesn't work for us in all places; this
switches to unsafe_memcpy() where appropriate to silence a few
warnings/errors.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/buckets.c

@@ -473,8 +473,9 @@ static inline int update_replicas_list(struct btree_trans *trans,
 	d = trans->fs_usage_deltas;
 	n = (void *) d->d + d->used;
 	n->delta = sectors;
-	memcpy((void *) n + offsetof(struct replicas_delta, r),
-	       r, replicas_entry_bytes(r));
+	unsafe_memcpy((void *) n + offsetof(struct replicas_delta, r),
+		      r, replicas_entry_bytes(r),
+		      "flexible array member embedded in strcuct with padding");
 	bch2_replicas_entry_sort(&n->r);
 	d->used += b;
 	return 0;

fs/bcachefs/extents.h

@@ -642,9 +642,11 @@ static inline void bch2_bkey_append_ptr(struct bkey_i *k, struct bch_extent_ptr
 
 		ptr.type = 1 << BCH_EXTENT_ENTRY_ptr;
 
-		memcpy((void *) &k->v + bkey_val_bytes(&k->k),
+		unsafe_memcpy((void *) &k->v + bkey_val_bytes(&k->k),
 			      &ptr,
-		       sizeof(ptr));
+			      sizeof(ptr),
+			      "Our memcpy target is relative to a zero size array ,"
+			      "compiler bounds checking doesn't work here");
 		k->k.u64s++;
 		break;
 	default: