ima: fix memory leak in ima_release_policy

When the "policy" securityfs file is opened for read, it is opened as a
sequential file. However, when it is eventually released, there is no
cleanup for the sequential file, therefore some memory is leaked.

This patch adds a call to seq_release() in ima_release_policy() to clean up
the memory when the file is opened for read.

Fixes: 80eae209 IMA: allow reading back the current policy
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Eric Richter <erichte@linux.vnet.ibm.com>
Tested-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>