SECURITY: selinux, fix update_rlimit_cpu parameter

Don't pass current RLIMIT_RTTIME to update_rlimit_cpu() in
selinux_bprm_committing_creds, since update_rlimit_cpu expects
RLIMIT_CPU limit.

Use proper rlim[RLIMIT_CPU].rlim_cur instead to fix that.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Acked-by: James Morris <jmorris@namei.org>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: Eric Paris <eparis@parisplace.org>
Cc: David Howells <dhowells@redhat.com>