Ecryptfs: Add mount option to check uid of device being mounted = expect uid

Ecryptfs: Add mount option to check uid of device being mounted = expect uid

commit 764355487ea220fdc2faf128d577d7f679b91f97 upstream.

Close a TOCTOU race for mounts done via ecryptfs-mount-private.  The mount
source (device) can be raced when the ownership test is done in userspace.
Provide Ecryptfs a means to force the uid check at mount time.

Signed-off-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: Tyler Hicks <tyhicks@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>