lib: zstd: Fix a warning in zstd_reset_dstream()

- ZSTD_resetDstream() is deprecated hence it is wise to make zstd_reset_dstream() functionally identical to ZSTD_resetDstream() as we have done for the compression stream in [1].
- Also make ZSTD_startingInputLength() visible for the decompression driver to be able to use it.
- This fixes the below warning:

../lib/zstd/zstd_decompress_module.c: In function 'zstd_reset_dstream':
../lib/zstd/zstd_decompress_module.c:80:9: warning: 'ZSTD_resetDStream' is deprecated [-Wdeprecated-declarations]
   80 |         return ZSTD_resetDStream(dstream);
      |         ^~~~~~
In file included from ../include/linux/zstd.h:26,
                 from ../lib/zstd/zstd_decompress_module.c:15:
../include/linux/zstd_lib.h:2569:27: note: declared here
 2569 | ZSTDLIB_STATIC_API size_t ZSTD_resetDStream(ZSTD_DStream* zds);
      |                           ^~~~~~~~~~~~~~~~~

[1]: facebook/zstd#3088

Signed-off-by: Cyber Knight <[email protected]>

include/linux/zstd_lib.h

@@ -2984,5 +2984,7 @@ ZSTDLIB_STATIC_API size_t ZSTD_decompressBlock(ZSTD_DCtx* dctx, void* dst, size_
 ZSTD_DEPRECATED("The block API is deprecated in favor of the normal compression API. See docs.")
 ZSTDLIB_STATIC_API size_t ZSTD_insertBlock    (ZSTD_DCtx* dctx, const void* blockStart, size_t blockSize);  /*< insert uncompressed block into `dctx` history. Useful for multi-blocks decompression. */
 
+ZSTDLIB_API size_t ZSTD_startingInputLength(ZSTD_format_e format);
+
 #endif   /* ZSTD_H_ZSTD_STATIC_LINKING_ONLY */
 

lib/zstd/decompress/zstd_decompress.c

@@ -223,7 +223,7 @@ size_t ZSTD_sizeof_DCtx (const ZSTD_DCtx* dctx)
 size_t ZSTD_estimateDCtxSize(void) { return sizeof(ZSTD_DCtx); }
 
 
-static size_t ZSTD_startingInputLength(ZSTD_format_e format)
+size_t ZSTD_startingInputLength(ZSTD_format_e format)
 {
     size_t const startingInputLength = ZSTD_FRAMEHEADERSIZE_PREFIX(format);
     /* only supports formats ZSTD_f_zstd1 and ZSTD_f_zstd1_magicless */

lib/zstd/zstd_decompress_module.c

@@ -15,6 +15,8 @@
 #include <linux/zstd.h>
 
 #include "common/zstd_deps.h"
+#include "common/error_private.h"  /* error codes and messages */
+#include "decompress/zstd_decompress_internal.h"   /* ZSTD_DCtx */
 
 /* Common symbols. zstd_compress must depend on zstd_decompress. */
 
@@ -77,7 +79,8 @@ EXPORT_SYMBOL(zstd_init_dstream);
 
 size_t zstd_reset_dstream(zstd_dstream *dstream)
 {
-	return ZSTD_resetDStream(dstream);
+        FORWARD_IF_ERROR(ZSTD_DCtx_reset(dstream, ZSTD_reset_session_only), "");
+        return ZSTD_startingInputLength(dstream->format);
 }
 EXPORT_SYMBOL(zstd_reset_dstream);