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: NRickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: NPaul Moore <pmoore@redhat.com>