eCryptfs: initialize payload_len in keystore.c

eCryptfs: initialize payload_len in keystore.c

commit fa5199648e273a5e3e80aca41c1eb53700438dc1 upstream.

This is meant to remove a compiler warning.  It should not make any
functional change.

payload_len should be initialized when it is passed to
write_tag_64_packet() as a pointer.  If that call fails, this function
should return early, and payload_len won't be used.

Signed-off-by: Simon Que <sque@chromium.org>
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>