ecryptfs: Fix memory leakage in keystore.c

commit 3edc8376c06133e3386265a824869cad03a4efd4 upstream.

In 'decrypt_pki_encrypted_session_key' function:

Initializes 'payload' pointer and releases it on exit.

Signed-off-by: Geyslan G. Bem <geyslan@gmail.com>
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Rui Xiang <rui.xiang@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>