eCryptfs: fixed bug in cipher handling

Cipher was sometimes not treated properly, causing valid
requests belonging to eCryptfs to be treated as regular.

Change-Id: Iabfb93cc4c9e9e167901043482eb99613ed70343
Signed-off-by: Andrey Markovytch <andreym@codeaurora.org>
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>