vsock/test: Introduce get_transports()

vsock/test: Introduce get_transports()

Return a bitmap of registered vsock transports. As guesstimated by grepping
/proc/kallsyms (CONFIG_KALLSYMS=y) for known symbols of type `struct
vsock_transport`, or `struct virtio_transport` in case the vsock_transport
is embedded within.

Note that the way `enum transport` and `transport_ksyms[]` are defined
triggers checkpatch.pl:

util.h:11: ERROR: Macros with complex values should be enclosed in parentheses
util.h:20: ERROR: Macros with complex values should be enclosed in parentheses
util.h:20: WARNING: Argument 'symbol' is not used in function-like macro
util.h:28: WARNING: Argument 'name' is not used in function-like macro

While commit 15d4734c7a58 ("checkpatch: qualify do-while-0 advice")
suggests it is known that the ERRORs heuristics are insufficient, I can not
find many other places where preprocessor is used in this
checkpatch-unhappy fashion. Notable exception being bcachefs, e.g.
fs/bcachefs/alloc_background_format.h. WARNINGs regarding unused macro
arguments seem more common, e.g. __ASM_SEL in arch/x86/include/asm/asm.h.

In other words, this might be unnecessarily complex. The same can be
achieved by just telling human to keep the order:

enum transport {
	TRANSPORT_LOOPBACK = BIT(0),
	TRANSPORT_VIRTIO = BIT(1),
	TRANSPORT_VHOST = BIT(2),
	TRANSPORT_VMCI = BIT(3),
	TRANSPORT_HYPERV = BIT(4),
	TRANSPORT_NUM = 5,
};

 #define KSYM_ENTRY(sym) "d " sym "_transport"

/* Keep `enum transport` order */
static const char * const transport_ksyms[] = {
	KSYM_ENTRY("loopback"),
	KSYM_ENTRY("virtio"),
	KSYM_ENTRY("vhost"),
	KSYM_ENTRY("vmci"),
	KSYM_ENTRY("vhs"),
};

Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Tested-by: Luigi Leonardi <leonardi@redhat.com>
Reviewed-by: Luigi Leonardi <leonardi@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20250611-vsock-test-inc-cov-v3-2-5834060d9c20@rbox.co
Signed-off-by: Jakub Kicinski <kuba@kernel.org>