LU-8196 obd: force very large allocations to use vmalloc

LU-8196 obd: force very large allocations to use vmalloc

In cases where the requsted allocation size is very large
use OBD_VMALLOC().  This avoids warnings from el7 and newer
kernels due to very large kmalloc()s.

Signed-off-by: Bob Glossman <bob.glossman@intel.com>
Change-Id: I87e95b1f8e5e50bc35d713a1775313c10f0368a9
Reviewed-on: http://review.whamcloud.com/20420
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Yang Sheng <yang.sheng@intel.com>
Tested-by: Jenkins
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>