rlimits: selinux, do rlimits changes under task_lock

When doing an exec, selinux updates rlimits in its code of current
process depending on current max. Make sure max or cur doesn't change
in the meantime by grabbing task_lock which do_prlimit needs for
changing limits too.

While at it, use rlimit helper for accessing CPU rlimit a line below.
To have a volatile access too.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Oleg Nesterov <oleg@redhat.com>