nfsd4: fix open downgrade, again

Yet another open-management regression:

	- nfs4_file_downgrade() doesn't remove the BOTH access bit on
	  downgrade, so the server's idea of the stateid's access gets
	  out of sync with the client's.  If we want to keep an O_RDWR
	  open in this case, we should do that in the file_put_access
	  logic rather than here.
	- We forgot to convert v4 access to an open mode here.

This logic has proven too hard to get right.  In the future we may
consider:
	- reexamining the lock/openowner relationship (locks probably
	  don't really need to take their own references here).
	- adding open upgrade/downgrade support to the vfs.
	- removing the atomic operations.  They're redundant as long as
	  this is all under some other lock.

Also, maybe some kind of additional static checking would help catch
O_/NFS4_SHARE_ACCESS confusion.

Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

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