bcachefs: Fix a valgrind conditional jump

Valgrind was complaining about a jump depending on uninitialized memory
- we weren't, but this change makes the code less confusing for valgrind
to follow.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/varint.c

@@ -97,7 +97,7 @@ int bch2_varint_encode_fast(u8 *out, u64 v)
 int bch2_varint_decode_fast(const u8 *in, const u8 *end, u64 *out)
 {
 	u64 v = get_unaligned_le64(in);
-	unsigned bytes = ffz(v & 255) + 1;
+	unsigned bytes = ffz(*in) + 1;
 
 	if (unlikely(in + bytes > end))
 		return -1;