selinux: wrap selinuxfs state

Move global selinuxfs state to a per-instance structure (selinux_fs_info),
and include a pointer to the selinux_state in this structure.
Pass this selinux_state to all security server operations, thereby
ensuring that each selinuxfs instance presents a view of and acts
as an interface to a particular selinux_state instance.

This change should have no effect on SELinux behavior or APIs
(userspace or LSM).  It merely wraps the selinuxfs global state,
links it to a particular selinux_state (currently always the single
global selinux_state) and uses that state for all operations.
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>