zstd: Fixing mixed module-builtin objects

With CONFIG_ZSTD_COMPRESS=m and CONFIG_ZSTD_DECOMPRESS=y we end up in
a situation when files from lib/zstd/common/ are compiled once to be
linked later for ZSTD_DECOMPRESS (build-in) and ZSTD_COMPRESS (module)
even though CFLAGS are different for builtins and modules.
So far somehow this was not a problem but enabling LLVM LTO exposes
the problem as:

ld.lld: error: linking module flags 'Code Model': IDs have conflicting values in 'lib/built-in.a(zstd_common.o at 5868)' and 'ld-temp.o'

This particular conflict is caused by KBUILD_CFLAGS=-mcmodel=medium vs.
KBUILD_CFLAGS_MODULE=-mcmodel=large , modules use the large model on
POWERPC as explained at
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/Makefile?h=v5.18-rc4#n127
but the current use of common files is wrong anyway.

This works around the issue by introducing a zstd_common module with
shared code.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

lib/zstd/common/zstd_common.c

@@ -13,6 +13,7 @@
 /*-*************************************
 *  Dependencies
 ***************************************/
+#include <linux/module.h>
 #define ZSTD_DEPS_NEED_MALLOC
 #include "zstd_deps.h"   /* ZSTD_malloc, ZSTD_calloc, ZSTD_free, ZSTD_memset */
 #include "error_private.h"
@@ -35,14 +36,17 @@ const char* ZSTD_versionString(void) { return ZSTD_VERSION_STRING; }
  *  tells if a return value is an error code
  *  symbol is required for external callers */
 unsigned ZSTD_isError(size_t code) { return ERR_isError(code); }
+EXPORT_SYMBOL_GPL(ZSTD_isError);
 
 /*! ZSTD_getErrorName() :
  *  provides error code string from function result (useful for debugging) */
 const char* ZSTD_getErrorName(size_t code) { return ERR_getErrorName(code); }
+EXPORT_SYMBOL_GPL(ZSTD_getErrorName);
 
 /*! ZSTD_getError() :
  *  convert a `size_t` function result into a proper ZSTD_errorCode enum */
 ZSTD_ErrorCode ZSTD_getErrorCode(size_t code) { return ERR_getErrorCode(code); }
+EXPORT_SYMBOL_GPL(ZSTD_getErrorCode);
 
 /*! ZSTD_getErrorString() :
  *  provides error code string from enum */
@@ -59,6 +63,7 @@ void* ZSTD_customMalloc(size_t size, ZSTD_customMem customMem)
         return customMem.customAlloc(customMem.opaque, size);
     return ZSTD_malloc(size);
 }
+EXPORT_SYMBOL_GPL(ZSTD_customMalloc);
 
 void* ZSTD_customCalloc(size_t size, ZSTD_customMem customMem)
 {
@@ -71,6 +76,7 @@ void* ZSTD_customCalloc(size_t size, ZSTD_customMem customMem)
     }
     return ZSTD_calloc(1, size);
 }
+EXPORT_SYMBOL_GPL(ZSTD_customCalloc);
 
 void ZSTD_customFree(void* ptr, ZSTD_customMem customMem)
 {
@@ -81,3 +87,7 @@ void ZSTD_customFree(void* ptr, ZSTD_customMem customMem)
             ZSTD_free(ptr);
     }
 }
+EXPORT_SYMBOL_GPL(ZSTD_customFree);
+
+MODULE_LICENSE("Dual BSD/GPL");
+MODULE_DESCRIPTION("Zstd Common");