RDMA/umem: Add a schedule point in ib_umem_get()

RDMA/umem: Add a schedule point in ib_umem_get()

Mapping as little as 64GB can take more than 10 seconds, triggering issues
on kernels with CONFIG_PREEMPT_NONE=y.

ib_umem_get() already splits the work in 2MB units on x86_64, adding a
cond_resched() in the long-lasting loop is enough to solve the issue.

Note that sg_alloc_table() can still use more than 100 ms, which is also
problematic. This might be addressed later in ib_umem_add_sg_table(),
adding new blocks in sgl on demand.

Link: https://lore.kernel.org/r/20200730015755.1827498-1-edumazet@google.com
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>