Fix OpenDowngrade for NFSv4.1 if a client sets the OPEN_SHARE_ACCESS_WANT* bits.

The NFSv4.1 RFC specifies that the OPEN_SHARE_ACCESS_WANT bits can be set
in the OpenDowngrade share_access argument and are basically ignored.
I do not know of a extant NFSv4.1 client that does this, but this little
patch fixes it just in case.
It also changes the error from NFSERR_BADXDR to NFSERR_INVAL since the NFSv4.1
RFC specifies this as the error to be returned if bogus bits are set.
(The NFSv4.0 RFC didn't specify any error for this, so the error reply can
be changed for NFSv4.0 as well.)
Found by inspection while looking at a problem with OpenDowngrade reported
for the ESXi 6.5 NFSv4.1 client.

Reported by: andreas.nagy@frequentis.com
PR: 227214
MFC after: 1 week