capability: Use current logging styles

Prefix logging output with "capability: " via pr_fmt.
Convert printks to pr_<level>.
Use pr_<level>_once instead of guard flags.
Coalesce formats.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>