ANDROID: disable BLK_DEV_NBD for allmodconfig

commit fad7cd33 ("nbd: add the check to prevent overflow in __nbd_ioctl()")
exposed something that's long been broken for semi-hosted environments
like the kernel in Clang:

check_mul_overflow() is implemented in terms of
__builtin_mul_overflow(). For 64b operands on 32b hosts, LLVM was
emitting libcalls to __mulodi4() which assumes that compiler-rt is being
linked against. The kernel does not do so, so LLVM was emitting calls to
functions that have no definition, resulting in the linkage failure:

ERROR: modpost: "__mulodi4" [drivers/block/nbd.ko] undefined!

I have been fixing LLVM upstream, see the six fixes linked from:
https://bugs.llvm.org/show_bug.cgi?id=28629#c23.

I still need to detect older toolchains that we'd still like to support,
then find an appropriate workaround for the kernel.

Disable network block devices for now, so that we don't lose coverage of
32b ARM allmodconfig builds which are currently red in our CI.

Bug: 199191028
Link: https://github.com/ClangBuiltLinux/linux/issues/1438


Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Change-Id: I79a597177f75370f60621b984cb8e21ca2a268d6