bpf: Remove migrate_{disable|enable} from bpf_selem_alloc()

bpf: Remove migrate_{disable|enable} from bpf_selem_alloc()

bpf_selem_alloc() has two callers:
(1) bpf_sk_storage_clone_elem()
bpf_sk_storage_clone() has already disabled migration before invoking
bpf_sk_storage_clone_elem().

(2) bpf_local_storage_update()
Its callers include: cgrp/task/inode/sock storage ->map_update_elem()
callbacks and bpf_{cgrp|task|inode|sk}_storage_get() helpers. These
running contexts have already disabled migration

Therefore, there is no need to add extra migrate_{disable|enable} pair
in bpf_selem_alloc().

Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20250108010728.207536-14-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>