lsm: cleanup initialize_lsm() and rename to lsm_init_single()

lsm: cleanup initialize_lsm() and rename to lsm_init_single()

Rename initialize_lsm() to be more consistent with the rest of the LSM
initialization changes and rework the function itself to better fit
with the "exit on fail" coding pattern.

Reviewed-by: Kees Cook <kees@kernel.org>
Reviewed-by: John Johansen <john.johansen@canonical.com>
Reviewed-by: Casey Schaufler <casey@schaufler-ca.com>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>