lsm: rename/rework append_ordered_lsm() into lsm_order_append()

lsm: rename/rework append_ordered_lsm() into lsm_order_append()

Rename append_ordered_lsm() to lsm_order_append() to better match
convention and do some rework.  The rework includes moving the
LSM_FLAG_EXCLUSIVE logic from lsm_prepare() to lsm_order_append()
in order to consolidate the individual LSM append/activation code,
and adding logic to skip appending explicitly disabled LSMs to the
active LSM list.

Reviewed-by: Casey Schaufler <casey@schaufler-ca.com>
Reviewed-by: John Johansen <john.johhansen@canonical.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>