lsm: cleanup the LSM blob size code

lsm: cleanup the LSM blob size code

Convert the lsm_blob_size fields to unsigned integers as there is no
current need for them to be negative, change "lsm_set_blob_size()" to
"lsm_blob_size_update()" to better reflect reality, and perform some
other minor cleanups to the associated code.

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>