arm64: errata: use KBUILD_CFLAGS_MODULE for erratum #843419

Commit df057cc7 ("arm64: errata: add module build workaround for
erratum #843419") sets CFLAGS_MODULE to ensure that the large memory
model is used by the compiler when building kernel modules.

However, CFLAGS_MODULE is an environment variable and intended to be
overridden on the command line, which appears to be the case with the
Ubuntu kernel packaging system, so use KBUILD_CFLAGS_MODULE instead.

Cc: <stable@vger.kernel.org>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Fixes: df057cc7 ("arm64: errata: add module build workaround for erratum #843419")
Reported-by: Dann Frazier <dann.frazier@canonical.com>
Tested-by: Dann Frazier <dann.frazier@canonical.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/Makefile

@@ -42,7 +42,7 @@ endif
 CHECKFLAGS	+= -D__aarch64__
 
 ifeq ($(CONFIG_ARM64_ERRATUM_843419), y)
-CFLAGS_MODULE	+= -mcmodel=large
+KBUILD_CFLAGS_MODULE	+= -mcmodel=large
 endif
 
 # Default value