MFC: r317382

MFC: r317382
Allow use of a write open stateid for reading in the NFSv4 server.

The NFSv4 RFCs give a server the option of allowing the use of an open
stateid for write access to be used for a Read operation.
This patch enables this by default and adds a sysctl to disable it,
for anyone who does not want this capability.
Allowing this is particularily useful for a pNFS Data Server (DS), since
they are not permitted to allow the use of special stateids.
Discovered during recent testing of the pNFS server under development.

git-svn-id: svn://svn.freebsd.org/base/stable/10@317986 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f