bpf: follow idr code convention

Generally we do a preload before doing idr allocation. This also help
improve the allocation success rate in memory pressure.

Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/bpf/syscall.c

@@ -203,11 +203,13 @@ static int bpf_map_alloc_id(struct bpf_map *map)
 {
 	int id;
 
+	idr_preload(GFP_KERNEL);
 	spin_lock_bh(&map_idr_lock);
 	id = idr_alloc_cyclic(&map_idr, map, 1, INT_MAX, GFP_ATOMIC);
 	if (id > 0)
 		map->id = id;
 	spin_unlock_bh(&map_idr_lock);
+	idr_preload_end();
 
 	if (WARN_ON_ONCE(!id))
 		return -ENOSPC;
@@ -940,11 +942,13 @@ static int bpf_prog_alloc_id(struct bpf_prog *prog)
 {
 	int id;
 
+	idr_preload(GFP_KERNEL);
 	spin_lock_bh(&prog_idr_lock);
 	id = idr_alloc_cyclic(&prog_idr, prog, 1, INT_MAX, GFP_ATOMIC);
 	if (id > 0)
 		prog->aux->id = id;
 	spin_unlock_bh(&prog_idr_lock);
+	idr_preload_end();
 
 	/* id is in [1, INT_MAX) */
 	if (WARN_ON_ONCE(!id))