bpf: add bpf_strcasecmp kfunc

bpf_strcasecmp() function performs same like bpf_strcmp() except ignoring
the case of the characters.

Signed-off-by: Rong Tao <rongtao@cestc.cn>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Viktor Malik <vmalik@redhat.com>
Link: https://lore.kernel.org/r/tencent_292BD3682A628581AA904996D8E59F4ACD06@qq.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>