LU-6261 gnilnd: Fix LIBCFS_ALLOC_POST incompatibility.

LU-6261 gnilnd: Fix LIBCFS_ALLOC_POST incompatibility.

master has changed LIBCFS_ALLOC to use vzalloc so LIBCFS_ALLOC_POST
does not contain a memset anymore.
kgnilnd_vmalloc() used LIBCFS_ALLOC_POST to zero memory allocated but
master removed the memset in favor of using vzalloc.
Use the __GFP_ZERO flag in (now named) kgnilnd_vzalloc to zero memory
allocated.

Signed-off-by: Chris Horn <hornc@cray.com>
Change-Id: Ia014c18f13e4e263eccb51d49d375fc4b3bc8b61
Reviewed-on: http://review.whamcloud.com/15435
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: James Shimek <jshimek@cray.com>
Reviewed-by: Doug Oucharek <doug.s.oucharek@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>