mm/hmm: return the fault type from hmm_pte_need_fault()

Using two bools instead of flags return is not necessary and leads to
bugs. Returning a value is easier for the compiler to check and easier to
pass around the code flow.

Convert the two bools into flags and push the change to all callers.

Link: https://lore.kernel.org/r/20200327200021.29372-3-jgg@ziepe.caReviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>