sparc32: Export vac_cache_size to fix build error

sparc32:allmodconfig fails to build with the following error.

ERROR: "vac_cache_size" [drivers/infiniband/sw/rxe/rdma_rxe.ko] undefined!

Fixes: cb886455

 ("infiniband: Fix alignment of mmap cookies ...")
Cc: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Cc: Doug Ledford <dledford@redhat.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>