eCryptfs: Remove ecryptfs_unlink_sigs warnings

A feature was added to the eCryptfs umount helper to automatically
unlink the keys used for an eCryptfs mount from the kernel keyring upon
umount.  This patch keeps the unrecognized mount option warnings for
ecryptfs_unlink_sigs out of the logs.

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