util: Make xmalloc_cacheline() allocate full cachelines.

util: Make xmalloc_cacheline() allocate full cachelines.

Until now, xmalloc_cacheline() has provided its caller memory that does not
share a cache line, but when posix_memalign() is not available it did not
provide a full cache line; instead, it returned memory that was offset 8
bytes into a cache line.  This makes it hard for clients to design
structures to be cache line-aligned.  This commit changes
xmalloc_cacheline() to always return a full cache line instead of memory
offset into one.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Bhanuprakash Bodireddy <Bhanuprakash.bodireddy@intel.com>
Tested-by: Bhanuprakash Bodireddy <Bhanuprakash.bodireddy@intel.com>
Tested-at: https://mail.openvswitch.org/pipermail/ovs-dev/2017-November/341362.html