bpf: drop refcount if bpf_map_new_fd() fails in map_create()

In bpf/syscall.c, map_create() first set map->usercnt to 1, a file
descriptor is supposed to return to userspace. When bpf_map_new_fd()
fails, drop the refcount.

Fixes: bd5f5f4 ("bpf: Add BPF_MAP_GET_FD_BY_ID")
Signed-off-by: Peng Sun <sironhide0null@gmail.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/bpf/syscall.c

@@ -559,12 +559,12 @@ static int map_create(union bpf_attr *attr)
 	err = bpf_map_new_fd(map, f_flags);
 	if (err < 0) {
 		/* failed to allocate fd.
-		 * bpf_map_put() is needed because the above
+		 * bpf_map_put_with_uref() is needed because the above
 		 * bpf_map_alloc_id() has published the map
 		 * to the userspace and the userspace may
 		 * have refcnt-ed it through BPF_MAP_GET_FD_BY_ID.
 		 */
-		bpf_map_put(map);
+		bpf_map_put_with_uref(map);
 		return err;
 	}