RDMA/umem: Fix signature of stub ib_umem_find_best_pgsz()

RDMA/umem: Fix signature of stub ib_umem_find_best_pgsz()

BugLink: https://bugs.launchpad.net/bugs/1902115
[ Upstream commit 61690d01db32eb1f94adc9ac2b8bb741d34e4671 ]

The original function returns unsigned long and 0 on failure.

Fixes: 4a35339958f1 ("RDMA/umem: Add API to find best driver supported page size in an MR")
Link: https://lore.kernel.org/r/0-v1-982a13cc5c6d+501ae-fix_best_pgsz_stub_jgg@nvidia.com
Reviewed-by: Gal Pressman <galpress@amazon.com>
Acked-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Ian May <ian.may@canonical.com>