LU-6763 lnet: test for sk_sleep presence in compact-2.6.h

LU-6763 lnet: test for sk_sleep presence in compact-2.6.h

Like Lustre external infiniband stacks create a compability
layer to handle various distributions and kernel versions.
Due to this besides the linux kernel sk_sleep can also be
defined by the external infiniband stack. We need to examine
the infiniband stack's headers to see if sk_sleep is available
there as well.

Change-Id: I0317349cfe13f281c56a8aec17dc330a0e3c759d
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: http://review.whamcloud.com/15386
Tested-by: Jenkins
Reviewed-by: Chris Horn <hornc@cray.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Amir Shehata <amir.shehata@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>