Given a large enough memory allocation, it is possible to wrap the
pinned_vm counter.  Check for addition overflow to prevent such
eventualities.

Fixes: 40ddacf2 ("RDMA/umem: Don't hold mmap_sem for too long")
Reported-by: NJason Gunthorpe <jgg@ziepe.ca>
Signed-off-by: NDoug Ledford <dledford@redhat.com>
Reviewed-by: NLeon Romanovsky <leonro@mellanox.com>
Signed-off-by: NJason Gunthorpe <jgg@mellanox.com>