eCryptfs: Print FNEK sig properly in /proc/mounts

The filename encryption key signature is not properly displayed in
/proc/mounts.  The "ecryptfs_sig=" mount option name is displayed for
all global authentication tokens, included those for filename keys.

This patch checks the global authentication token flags to determine if
the key is a FEKEK or FNEK and prints the appropriate mount option name
before the signature.

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