RDMA/odp: Add missing cast for 32 bit

RDMA/odp: Add missing cast for 32 bit

length is a size_t which is unsigned int on 32 bit:

../drivers/infiniband/core/umem_odp.c: In function 'ib_init_umem_odp':
../include/linux/overflow.h:59:15: warning: comparison of distinct pointer types lacks a cast
   59 |  (void) (&__a == &__b);   \
      |               ^~
../drivers/infiniband/core/umem_odp.c:220:7: note: in expansion of macro 'check_add_overflow'

Fixes: 204e3e5630c5 ("RDMA/odp: Check for overflow when computing the umem_odp end")
Link: https://lore.kernel.org/r/20190908080726.30017-1-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>