kernel: backport get_cycles() fix

get_cycles() is used in some places as one part of the seed for the
kernel PRNG.

This backports the following commit from mainline linux kernel:
9c9b415c50bc298ac61412dff856eae2f54889ee

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

SVN-Revision: 38834