selinux: reconcile security_netlbl_secattr_to_sid() and mls_import_netlbl_cat()

Move the NetLabel secattr MLS category import logic into
mls_import_netlbl_cat() where it belongs, and use the
mls_import_netlbl_cat() function in security_netlbl_secattr_to_sid().

Reported-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Paul Moore <pmoore@redhat.com>