MFC: r317345

MFC: r317345
Make the NFSv4 client to use a write open for reading if allowed by the server.

An NFSv4 server has the option of allowing a Read to be done using a Write
Open. If this is not allowed, the server will return NFSERR_OPENMODE.
This patch attempts the read with a write open and then disables this
if the server replies NFSERR_OPENMODE.
This change will avoid some uses of the special stateids. This will be
useful for pNFS/DS Reads, since they cannot use special stateids.
It will also be useful for any NFSv4 server that does not support reading
via the special stateids. It has been tested against both types of NFSv4 server.

git-svn-id: svn://svn.freebsd.org/base/stable/10@317984 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f