avoid symbol collission with in-kernel zstdlib

in case zfs is compiled as in kernel static variant
and the in kernel zstd library is compiled static into the kernel, a
symbol collision will occur. this wrapper header does rename all
relevant zstd function to avoid this problem

Signed-off-by: Sebastian Gottschall <s.gottschall@dd-wrt.com>

lib/libzstd/Makefile.am

@@ -15,8 +15,11 @@ nodist_libzstd_la_SOURCES = $(KERNEL_C)
 
 # -fno-tree-vectorize is set for gcc in zstd/common/compiler.h
 # Set it for other compilers, too.
-lib/zstd.$(OBJEXT):  CFLAGS += -fno-tree-vectorize
-lib/zstd.l$(OBJEXT): CFLAGS += -fno-tree-vectorize
+lib/zstd.$(OBJEXT):  CFLAGS += -fno-tree-vectorize -include $(top_srcdir)/module/zstd/include/zstd_compat_wrapper.h -Wp,-w
+lib/zstd.l$(OBJEXT): CFLAGS += -fno-tree-vectorize -include $(top_srcdir)/module/zstd/include/zstd_compat_wrapper.h -Wp,-w
+
+zfs_zstd.$(OBJEXT):  CFLAGS += -include $(top_srcdir)/module/zstd/include/zstd_compat_wrapper.h
+zfs_zstd.l$(OBJEXT): CFLAGS += -include $(top_srcdir)/module/zstd/include/zstd_compat_wrapper.h
 
 # Quiet warnings about frame size due to unused code in unmodified zstd lib
 lib/zstd.$(OBJEXT):  CFLAGS += -Wframe-larger-than=20480

module/zstd/Makefile.in

@@ -24,7 +24,9 @@ $(obj)/lib/zstd.o: c_flags += -fno-tree-vectorize
 $(obj)/lib/zstd.o: c_flags += -Wframe-larger-than=20480
 
 # Disable aarch64 neon SIMD instructions for kernel mode
-$(obj)/lib/zstd.o: c_flags += -include $(zstd_include)/aarch64_compat.h
+$(obj)/lib/zstd.o: c_flags += -include $(zstd_include)/aarch64_compat.h -include $(zstd_include)/zstd_compat_wrapper.h -Wp,-w
+
+$(obj)/zfs_zstd.o: c_flags += -include $(zstd_include)/zstd_compat_wrapper.h
 
 $(MODULE)-objs += zfs_zstd.o
 $(MODULE)-objs += lib/zstd.o