bpf: add bpf_strcasestr,bpf_strncasestr kfuncs

[ Upstream commit b5b693f73589283e75436e64fab46bf6298abd89 ]

bpf_strcasestr() and bpf_strncasestr() functions perform same like
bpf_strstr() and bpf_strnstr() except ignoring the case of the
characters.

Signed-off-by: Rong Tao <rongtao@cestc.cn>
Link: https://lore.kernel.org/r/tencent_B01165355D42A8B8BF5E8D0A21EE1A88090A@qq.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>