lsm: get rid of the lsm_names list and do some cleanup

lsm: get rid of the lsm_names list and do some cleanup

The LSM currently has a lot of code to maintain a list of the currently
active LSMs in a human readable string, with the only user being the
"/sys/kernel/security/lsm" code.  Let's drop all of that code and
generate the string on first use and then cache it for subsequent use.

Signed-off-by: Paul Moore <paul@paul-moore.com>