bpf: Add zero_map_value to zero map value with special fields

We need this helper to skip over special fields (bpf_spin_lock,
bpf_timer, kptrs) while zeroing a map value. Use the same logic as
copy_map_value but memset instead of memcpy.

Currently, the code zeroing map value memory does not have to deal with
special fields, hence this is a prerequisite for introducing such
support.
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220904204145.3089-4-memxor@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf.h

@@ -329,6 +329,25 @@ static inline void copy_map_value_long(struct bpf_map *map, void *dst, void *src
 	__copy_map_value(map, dst, src, true);
 }
 
+static inline void zero_map_value(struct bpf_map *map, void *dst)
+{
+	u32 curr_off = 0;
+	int i;
+
+	if (likely(!map->off_arr)) {
+		memset(dst, 0, map->value_size);
+		return;
+	}
+
+	for (i = 0; i < map->off_arr->cnt; i++) {
+		u32 next_off = map->off_arr->field_off[i];
+
+		memset(dst + curr_off, 0, next_off - curr_off);
+		curr_off += map->off_arr->field_sz[i];
+	}
+	memset(dst + curr_off, 0, map->value_size - curr_off);
+}
+
 void copy_map_value_locked(struct bpf_map *map, void *dst, void *src,
 			   bool lock_src);
 void bpf_timer_cancel_and_free(void *timer);