eCryptfs: fixed some major bugs

1. Fixed bug which didn't allow several threads to work simultaneously
on files in eCryptfs mounted folder

2. Fixed bug where PFK close callback was invoked multiple times when
files was opened and closed multiple times. Now it is invoked just once
when files is closed for the last time

Change-Id: Iaa3ada03500e5a12752918b5d2bb4a852ddca5f0
Signed-off-by: Andrey Markovytch <andreym@codeaurora.org>