fs_mgr: fix encryptable=footer support

If the userdata partition has the encryptable=footer fstab option,
fs_mgr must leave room for the crypt footer.

Change-Id: Id07818c5d93aafc27577f72fb0a780f26db51b16
Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>