RDMA/siw: Fix user page pinning accounting

RDMA/siw: Fix user page pinning accounting

To avoid racing with other user memory reservations, immediately
account full amount of pages to be pinned.

Fixes: 2251334dcac9 ("rdma/siw: application buffer management")
Reported-by: Jason Gunthorpe <jgg@nvidia.com>
Suggested-by: Alistair Popple <apopple@nvidia.com>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Bernard Metzler <bmt@zurich.ibm.com>
Link: https://lore.kernel.org/r/20230202101000.402990-1-bmt@zurich.ibm.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>