[Nouveau] hmm_range_fault related fixes and legacy API removal v3

[Christoph Hellwig]

Hi Jérôme, Ben and Jason,

below is a series against the hmm tree which fixes up the mmap_sem
locking in nouveau and while at it also removes leftover legacy HMM APIs
only used by nouveau.

The first 4 patches are a bug fix for nouveau, which I suspect should
go into this merge window even if the code is marked as staging, just
to avoid people copying the breakage.

Changes since v2:
 - new patch from Jason to document FAULT_FLAG_ALLOW_RETRY semantics
   better
 - remove -EAGAIN handling in nouveau earlier

Changes since v1:
 - don't return the valid state from hmm_range_unregister
 - additional nouveau cleanups