ima: change the default hash algorithm to SHA1 in ima_eventdigest_ng_init()

Replace HASH_ALGO__LAST with HASH_ALGO_SHA1 as the initial value of
the hash algorithm so that the prefix 'sha1:' is added to violation
digests.

Fix commit:
  4d7aeee7 ima: define new template ima-ng and template fields d-ng and n-ng

Signed-off-by: Roberto Sassu <roberto.sassu@polito.it>
Cc: <stable@vger.kernel.org> # 3.13.x
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>