LU-11224 obd: use correct ip_compute_csum() version

LU-11224 obd: use correct ip_compute_csum() version

The linux kernel provides a generic platform independent version
of ip_compute_csum() as well as platform optimized versions. Some
platforms will disable the generic platform version in favor of
the optimized one. If the generic version is disabled and if the
checksum.h header from asm-generic is used then we will end up
with a undefined symbol error when loading the obdclass module.
The solution is to use the platform specific checksum.h header
that will handle using the generic or optimized version for us.
As a bounus we get better performance with the right kernel
configuration.

Test-Parameters: trivial

Change-Id: Ia0cfc9f4363bb61d5e381790655423ff5f91d9be
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/32953
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>