ima: annotate iint mutex to avoid lockdep false positive warnings

It is not clear that IMA should be nested at all, but as long is it
measures files both on overlayfs and on underlying fs, we need to
annotate the iint mutex to avoid lockdep false positives related to
IMA + overlayfs, same as overlayfs annotates the inode mutex.

Reported-and-tested-by:  <syzbot+b42fe626038981fb7bfa@syzkaller.appspotmail.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>