lib: zstd: Refactor intentional wrap-around test

In an effort to separate intentional arithmetic wrap-around from
unexpected wrap-around, we need to refactor places that depend on this
kind of math. One of the most common code patterns of this is:

	VAR + value < VAR

Notably, this is considered "undefined behavior" for signed and pointer
types, which the kernel works around by using the -fno-strict-overflow
option in the build[1] (which used to just be -fwrapv). Regardless, we
want to get the kernel source to the position where we can meaningfully
instrument arithmetic wrap-around conditions and catch them when they
are unexpected, regardless of whether they are signed[2], unsigned[3],
or pointer[4] types.

Switch to a more regular type for a 64-bit value and refactor the
open-coded wrap-around addition test to use subtraction from the type max
(since add_would_overflow() may not be defined in early boot code). This
paves the way to enabling the wrap-around sanitizers in the future.

Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
Link: KSPP#26 [2]
Link: KSPP#27 [3]
Link: KSPP#344 [4]
Cc: Nick Terrell <terrelln@fb.com>
Cc: Paul Jones <paul@pauljones.id.au>
Cc: Sedat Dilek <sedat.dilek@gmail.com>
Cc: Oleksandr Natalenko <oleksandr@natalenko.name>
Cc: Xin Gao <gaoxin@cdjrlc.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

lib/zstd/decompress/zstd_decompress.c

@@ -618,7 +618,7 @@ size_t ZSTD_readSkippableFrame(void* dst, size_t dstCapacity,
  * @return : decompressed size of the frames contained */
 unsigned long long ZSTD_findDecompressedSize(const void* src, size_t srcSize)
 {
-    unsigned long long totalDstSize = 0;
+    U64 totalDstSize = 0;
 
     while (srcSize >= ZSTD_startingInputLength(ZSTD_f_zstd1)) {
         U32 const magicNumber = MEM_readLE32(src);
@@ -636,7 +636,7 @@ unsigned long long ZSTD_findDecompressedSize(const void* src, size_t srcSize)
         {   unsigned long long const fcs = ZSTD_getFrameContentSize(src, srcSize);
             if (fcs >= ZSTD_CONTENTSIZE_ERROR) return fcs;
 
-            if (totalDstSize + fcs < totalDstSize)
+            if (U64_MAX - totalDstSize < fcs)
                 return ZSTD_CONTENTSIZE_ERROR; /* check for overflow */
             totalDstSize += fcs;
         }