mm + fs: extends support for cache dropping

Exposes drop_pagecache_sb (required by eCryptfs cache wiping)
Adds truncate_inode_pages_fill_zero (required by eCryptfs cache wiping),
which not only truncates pages but also fills them with 0, so that the
cached data can no longer be retrieved.

Change-Id: Icfc18a2c8cdc922e71ee17add6459a1355e77ba6
Signed-off-by: Andrey Markovytch <andreym@codeaurora.org>