textsearch: support for case insensitive searching

The function textsearch_prepare has a new flag to support case
insensitive searching.

Signed-off-by: Joonwoo Park <joonwpark81@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>