eCryptfs: Fix payload_len unitialized variable warning

fs/ecryptfs/keystore.c: In function ‘ecryptfs_generate_key_packet_set’:
fs/ecryptfs/keystore.c:1991:28: warning: ‘payload_len’ may be used uninitialized in this function [-Wuninitialized]
fs/ecryptfs/keystore.c:1976:9: note: ‘payload_len’ was declared here

Signed-off-by: Tyler Hicks <tyhicks@linux.vnet.ibm.com>