bpf: Fix BPF_LSM kconfig symbol dependency

Similarly as 6bdacdb4 ("bpf: Fix BPF_JIT kconfig symbol dependency") we
need to detangle the hard BPF_LSM dependency on NET. This was previously
implicit by its dependency on BPF_JIT which itself was dependent on NET (but
without any actual/real hard dependency code-wise). Given the latter was
lifted, so should be the former as BPF_LSMs could well exist on net-less
systems. This therefore also fixes a randconfig build error recently reported
by Randy:

  ld: kernel/bpf/bpf_lsm.o: in function `bpf_lsm_func_proto':
  bpf_lsm.c:(.text+0x1a0): undefined reference to `bpf_sk_storage_get_proto'
  ld: bpf_lsm.c:(.text+0x1b8): undefined reference to `bpf_sk_storage_delete_proto'
  [...]

Fixes: b24abcff ("bpf, kconfig: Add consolidated menu entry for bpf with core options")
Reported-by: NRandy Dunlap <rdunlap@infradead.org>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Acked-by: NRandy Dunlap <rdunlap@infradead.org>
Tested-by: NRandy Dunlap <rdunlap@infradead.org>

kernel/bpf/bpf_lsm.c

@@ -107,10 +107,12 @@ bpf_lsm_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_inode_storage_get_proto;
 	case BPF_FUNC_inode_storage_delete:
 		return &bpf_inode_storage_delete_proto;
+#ifdef CONFIG_NET
 	case BPF_FUNC_sk_storage_get:
 		return &bpf_sk_storage_get_proto;
 	case BPF_FUNC_sk_storage_delete:
 		return &bpf_sk_storage_delete_proto;
+#endif /* CONFIG_NET */
 	case BPF_FUNC_spin_lock:
 		return &bpf_spin_lock_proto;
 	case BPF_FUNC_spin_unlock: