sched: Move code around

In preparation to reworking set_cpus_allowed_ptr() move some code
around. This also removes some superfluous #ifdefs and adds comments
to some #endifs.

   text    data     bss     dec     hex filename
12211532        1738144 1081344 15031020         e55aec defconfig-build/vmlinux.pre
12211532        1738144 1081344 15031020         e55aec defconfig-build/vmlinux.post

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: ktkhai@parallels.com
Cc: rostedt@goodmis.org
Cc: juri.lelli@gmail.com
Cc: pang.xunlei@linaro.org
Cc: oleg@redhat.com
Cc: wanpeng.li@linux.intel.com
Cc: umgwanakikbuti@gmail.com
Link: http://lkml.kernel.org/r/20150611124743.662086684@infradead.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>