mm/hmm: do not set pfns when returning an error code

mm/hmm: do not set pfns when returning an error code

Most places that return an error code, like -EFAULT, do not set
HMM_PFN_ERROR, only two places do this.

Resolve this inconsistency by never setting the pfns on an error
exit. This doesn't seem like a worthwhile thing to do anyhow.

If for some reason it becomes important, it makes more sense to directly
return the address of the failing page rather than have the caller scan
for the HMM_PFN_ERROR.

No caller inspects the pnfs output array if hmm_range_fault() fails.

Link: https://lore.kernel.org/r/20200327200021.29372-9-jgg@ziepe.ca
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>