lsm,selinux: add new hook to compare new mount to an existing mount

Add a new hook that takes an existing super block and a new mount
with new options and determines if new options confict with an
existing mount or not.

A filesystem can use this new hook to determine if it can share
the an existing superblock with a new superblock for the new mount.

Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Acked-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
[PM: tweak the subject line, fix tab/space problems]
Signed-off-by: Paul Moore <paul@paul-moore.com>