bcachefs: Fix some memcpy() warnings

With CONFIG_FORTIFY_SOURCE, the compiler attempts to warn about mempcys
that extend past struct field boundaries. This results in some spurious
warnings where we use embedded variable length structs, this patch
switches to unsafe_mecpy() to fix the warnings.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/buckets.c

@@ -156,7 +156,8 @@ struct bch_fs_usage_online *bch2_fs_usage_read(struct bch_fs *c)
 
 	do {
 		seq = read_seqcount_begin(&c->usage_lock);
-		memcpy(&ret->u, c->usage_base, u64s * sizeof(u64));
+		unsafe_memcpy(&ret->u, c->usage_base, u64s * sizeof(u64),
+			      "embedded variable length struct");
 		for (i = 0; i < ARRAY_SIZE(c->usage); i++)
 			acc_u64s_percpu((u64 *) &ret->u, (u64 __percpu *) c->usage[i], u64s);
 	} while (read_seqcount_retry(&c->usage_lock, seq));