eCryptfs: Fix payload_len unitialized variable warning

fs/ecryptfs/keystore.c: In function ‘ecryptfs_generate_key_packet_set’:
fs/ecryptfs/keystore.c:1168:28: warning: 'payload_len' may be used uninitialized

Change-Id: Ia64fbb5705eb61cb8136553040c27aed76527d8c