lib/ts_bm: reset initial match offset for every block of text

lib/ts_bm: reset initial match offset for every block of text

The `shift` variable which indicates the offset in the string at which
to start matching the pattern is initialized to `bm->patlen - 1`, but it
is not reset when a new block is retrieved.  This means the implemen-
tation may start looking at later and later positions in each successive
block and miss occurrences of the pattern at the beginning.  E.g.,
consider a HTTP packet held in a non-linear skb, where the HTTP request
line occurs in the second block:

  [... 52 bytes of packet headers ...]
  GET /bmtest HTTP/1.1\r\nHost: www.example.com\r\n\r\n

and the pattern is "GET /bmtest".

Once the first block comprising the packet headers has been examined,
`shift` will be pointing to somewhere near the end of the block, and so
when the second block is examined the request line at the beginning will
be missed.

Reinitialize the variable for each new block.

Fixes: 8082e4ed0a61 ("[LIB]: Boyer-Moore extension for textsearch infrastructure strike #2")
Link: https://bugzilla.netfilter.org/show_bug.cgi?id=1390
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>