MFC: r307694
A problem w.r.t. interoperation between the FreeBSD NFSv4.1 server with
delegations enabled and the Linux NFSv4.1 client was reported in
reviews.freebsd.org/D7891.
I believe that the FreeBSD server behaviour conforms to the RFC and that
the Linux client has a bug. Therefore, I do not think the proposed patch
is appropriate. When nfsrv_writedelegifpos is non-zero, the FreeBSD
server will issue a write delegation for a read open if possible.
The Linux client then erroneously assumes that the credentials used for
the read open can write the file.
This patch reverses the default value for nfsrv_writedelegifpos to 0 so
that the default behaviour is Linux compatible and adds a sysctl that can
be used to set nfsrv_writedelegifpos.

This change should only affect users that are mounting a FreeBSD server
with delegations enabled (they are not enabled by default) with a Linux
NFSv4.1 client mount.