textsearch: fix Boyer-Moore text search bug

The current logic has a bug which cannot find matching pattern, if the
pattern is matched from the first character of target string.
for example:
	pattern=abc, string=abcdefg
	pattern=a,   string=abcdefg
Searching algorithm should return 0 for those things.

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>