bpf: add 'flags' attribute to BPF_MAP_UPDATE_ELEM command

the current meaning of BPF_MAP_UPDATE_ELEM syscall command is:
either update existing map element or create a new one.
Initially the plan was to add a new command to handle the case of
'create new element if it didn't exist', but 'flags' style looks
cleaner and overall diff is much smaller (more code reused), so add 'flags'
attribute to BPF_MAP_UPDATE_ELEM command with the following meaning:
 #define BPF_ANY	0 /* create new element or update existing */
 #define BPF_NOEXIST	1 /* create new element if it didn't exist */
 #define BPF_EXIST	2 /* update existing element */

bpf_update_elem(fd, key, value, BPF_NOEXIST) call can fail with EEXIST
if element already exists.

bpf_update_elem(fd, key, value, BPF_EXIST) can fail with ENOENT
if element doesn't exist.

Userspace will call it as:
int bpf_update_elem(int fd, void *key, void *value, __u64 flags)
{
    union bpf_attr attr = {
        .map_fd = fd,
        .key = ptr_to_u64(key),
        .value = ptr_to_u64(value),
        .flags = flags;
    };

    return bpf(BPF_MAP_UPDATE_ELEM, &attr, sizeof(attr));
}

First two bits of 'flags' are used to encode style of bpf_update_elem() command.
Bits 2-63 are reserved for future use.

Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/bpf/syscall.c

@@ -190,7 +190,7 @@ err_put:
 	return err;
 }
 
-#define BPF_MAP_UPDATE_ELEM_LAST_FIELD value
+#define BPF_MAP_UPDATE_ELEM_LAST_FIELD flags
 
 static int map_update_elem(union bpf_attr *attr)
 {
@@ -231,7 +231,7 @@ static int map_update_elem(union bpf_attr *attr)
 	 * therefore all map accessors rely on this fact, so do the same here
 	 */
 	rcu_read_lock();
-	err = map->ops->map_update_elem(map, key, value);
+	err = map->ops->map_update_elem(map, key, value, attr->flags);
 	rcu_read_unlock();
 
 free_value: