sys: Disable -Walloc-size-larger-than for GCC 9+

By default this warns about sizes larger than PTRDIFF_MAX passed to
malloc (rather than SIZE_MAX).  This doesn't trigger
deterministically, but it does trigger for kmalloc() of struct_size()
in iwlwifi's iwl_configure_rxq even when struct_size() is changed to
use PTRDIFF_MAX.  NB: struct_size() in Linux caps the size at
SIZE_MAX, not PTRDIFF_MAX via size_mul().

Reviewed by:	imp
Differential Revision:	https://reviews.freebsd.org/D42587

sys/conf/kern.mk

@@ -76,6 +76,7 @@ CWARNEXTRA+=	-Wno-error=packed-not-aligned
 .endif
 .if ${COMPILER_VERSION} >= 90100
 CWARNEXTRA+=	-Wno-address-of-packed-member			\
+		-Wno-alloc-size-larger-than			\
 		-Wno-error=alloca-larger-than=
 .if ${COMPILER_VERSION} >= 120100
 CWARNEXTRA+=	-Wno-error=nonnull				\