textsearch: ts_fsm: return error on request for case insensitive search

For fsm text search, handle case insensitive parameter as -EINVAL.

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>