bpf: Disallow BPF_PRELOAD in allmodconfig builds

The CC_CAN_LINK checks that the host compiler can link, but bpf_preload
relies on libbpf which in turn needs libelf to be present during linking.
allmodconfig runs in odd setups with cross compilers and missing host
libraries like libelf. Instead of extending kconfig with every possible
library that bpf_preload might need disallow building BPF_PRELOAD in
such build-only configurations.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/preload/Kconfig

@@ -6,6 +6,9 @@ config USERMODE_DRIVER
 menuconfig BPF_PRELOAD
 	bool "Preload BPF file system with kernel specific program and map iterators"
 	depends on BPF
+	# The dependency on !COMPILE_TEST prevents it from being enabled
+	# in allmodconfig or allyesconfig configurations
+	depends on !COMPILE_TEST
 	select USERMODE_DRIVER
 	help
 	  This builds kernel module with several embedded BPF programs that are