bcachefs: varint: Avoid left-shift of a negative value

Shifting a negative value left is undefined.
Signed-off-by: Tavian Barnes <tavianator@tavianator.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/varint.c

@@ -85,7 +85,7 @@ int bch2_varint_encode_fast(u8 *out, u64 v)
 
 	if (likely(bytes < 9)) {
 		v <<= bytes;
-		v |= ~(~0 << (bytes - 1));
+		v |= ~(~0U << (bytes - 1));
 	} else {
 		*out++ = 255;
 		bytes = 9;