bpf: test for valid type in bpf_get_work_dir

Jan-Erik reported an assertion in bpf_prog_to_subdir() failed where
type was BPF_PROG_TYPE_UNSPEC, which is only used in bpf_init_env()
to auto-mount and cache the bpf fs mount point.

Therefore, make sure when bpf_init_env() is called multiple times
(f.e. eBPF classifier with eBPF action attached) and bpf_mnt_cached
is set already that the type is also valid. In bpf_init_env(), we're
only interested in the mount point and not a type-specific subdir.

Fixes: e42256699c ("bpf: make tc's bpf loader generic and move into lib")
Reported-by: Jan-Erik Rediger <janerik@rediger.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

lib/bpf.c

@@ -596,7 +596,7 @@ static const char *bpf_get_work_dir(enum bpf_prog_type type)
 	if (bpf_mnt_cached) {
 		const char *out = mnt;
 
-		if (out) {
+		if (out && type) {
 			snprintf(bpf_tmp, sizeof(bpf_tmp), "%s%s/",
 				 out, bpf_prog_to_subdir(type));
 			out = bpf_tmp;