nouveau/hmm: fault one page at a time

The SVM page fault handler groups faults into a range of contiguous
virtual addresses and requests hmm_range_fault() to populate and return
the page frame number of system memory mapped by the CPU.  In preparation
for supporting large pages to be mapped by the GPU, process faults one
page at a time. In addition, use the hmm_range default_flags to fix a
corner case where the input hmm_pfns array is not reinitialized after
hmm_range_fault() returns -EBUSY and must be called again.

Link: https://lore.kernel.org/r/20200701225352.9649-2-rcampbell@nvidia.com


Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>