root_plug: use cap_task_prctl

With the introduction of per-process securebits, the capabilities-related
prctl callbacks were moved into cap_task_prctl().  Have root_plug use
cap_task_prctl() so that PR_SET_KEEPCAPS is defined.

Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>