security: clarify cap_inode_getsecctx description

Make it clear that cap_inode_getsecctx shouldn't return success without
filling in the context data.

Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>