nouveau: return -EBUSY when hmm_range_wait_until_valid fails

-EAGAIN has a magic meaning for non-blocking faults, so don't overload it.
Given that the caller doesn't check for specific error codes this change
is purely cosmetic.

Link: https://lore.kernel.org/r/20190724065258.16603-6-hch@lst.de


Tested-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>