Silence unused-but-set-variable warnings

Clang 13.0.0 added support for `Wunused-but-set-parameter` and
`-Wunused-but-set-variable` which correctly detects two unused
variables in zstd resulting in a build failure.  This commit
annotates these instances accordingly.

  https://releases.llvm.org/13.0.1/tools/clang/docs/ReleaseNotes.html#id6

In FSE_createCTable(), malloc() is intentionally defined as NULL when
compiled in the kernel so the variable is unused.

  zstd/lib/compress/fse_compress.c:307:12: error: variable 'size'
  set but not used [-Werror,-Wunused-but-set-variable]

Additionally, in ZSTD_seqDecompressedSize() the assert is compiled
out similarly resulting in an unused variable.

  zstd/lib/compress/zstd_compress_superblock.c:412:12: error: variable
  'litLengthSum' set but not used [-Werror,-Wunused-but-set-variable]

Reviewed-by: Rich Ercolani <rincebrain@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13382

module/zstd/lib/compress/fse_compress.c

@@ -304,7 +304,7 @@ size_t FSE_writeNCount (void* buffer, size_t bufferSize,
 
 FSE_CTable* FSE_createCTable (unsigned maxSymbolValue, unsigned tableLog)
 {
-    size_t size;
+    size_t size __attribute__ ((unused));
     if (tableLog > FSE_TABLELOG_ABSOLUTE_MAX) tableLog = FSE_TABLELOG_ABSOLUTE_MAX;
     size = FSE_CTABLE_SIZE_U32 (tableLog, maxSymbolValue) * sizeof(U32);
     return (FSE_CTable*)malloc(size);

module/zstd/lib/compress/zstd_compress_superblock.c

@@ -409,7 +409,7 @@ static size_t ZSTD_seqDecompressedSize(seqStore_t const* seqStore, const seqDef*
     const seqDef* const send = sequences + nbSeq;
     const seqDef* sp = sstart;
     size_t matchLengthSum = 0;
-    size_t litLengthSum = 0;
+    size_t litLengthSum __attribute__ ((unused)) = 0;
     while (send-sp > 0) {
         ZSTD_sequenceLength const seqLen = ZSTD_getSequenceLength(seqStore, sp);
         litLengthSum += seqLen.litLength;