eCryptfs: fixes issue where files sometimes got corrupted upon close

Fixes implementation of cache cleaning, it was implemented using outdated
code which cleaned valid pages as well by mistake. The fix uses the
original implementation from mm layer which was tweaked to zero pages
after releasing them. Also it is now invoked only when there is HW
encryption (meaning that cache is unencrypted)

Change-Id: If713fd17a89be6d998c1a7ba86bee4c17d2bca0f
Signed-off-by: Andrey Markovytch <andreym@codeaurora.org>