selftests/bpf: Allow appending to guest kernel cmdline in vmtest.sh

vmtest.sh currently hardcodes the guest command line, so there is no
way to ask for such a setting without editing the script. Append
$KERNEL_CMDLINE_EXTRA when set so it can be used for testing the
BPF keyring:

   # KERNEL_CMDLINE_EXTRA="bpf.keyring_unsealed=1" \
	./vmtest.sh -- ./test_progs -t signed_loader

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20260828175227.1537793-10-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/vmtest.sh

@@ -107,6 +107,14 @@ Options:
 	-s)		Instead of powering off the VM, start an interactive
 			shell. If <command> is specified, the shell runs after
 			the command finishes executing
+
+Environment variables:
+
+	KERNEL_CMDLINE_EXTRA
+			Extra arguments to append to the guest kernel command
+			line, for tests that need a boot-time setting. e.g:
+
+	  KERNEL_CMDLINE_EXTRA="bpf.keyring_unsealed=1" $0 -- ./test_progs -t signed_loader
 EOF
 }
 
@@ -286,6 +294,12 @@ EOF
 		QEMU_FLAGS=("${HOST_FLAGS[@]}")
 	fi
 
+	local kernel_cmdline="root=/dev/vda rw console=${QEMU_CONSOLE}"
+
+	if [[ -n "${KERNEL_CMDLINE_EXTRA:-}" ]]; then
+		kernel_cmdline+=" ${KERNEL_CMDLINE_EXTRA}"
+	fi
+
 	${QEMU_BINARY} \
 		-nodefaults \
 		-display none \
@@ -294,7 +308,7 @@ EOF
 		-m 4G \
 		-drive file="${rootfs_img}",format=raw,index=1,media=disk,if=virtio,cache=none \
 		-kernel "${kernel_bzimage}" \
-		-append "root=/dev/vda rw console=${QEMU_CONSOLE}"
+		-append "${kernel_cmdline}"
 }
 
 copy_logs()