Luoqi Chen reported a problem on freebsd-fs@ where a Linux NFSv4 client
was able to open and write to a file when the file's permissions were
not set to allow the owner write access.

Since NFS servers check file permissions on every write RPC, it is standard
practice to allow the owner of the file to do writes, regardless of
file permissions. This provides POSIX like behaviour, since POSIX only
checks permissions upon open(2).
The traditional way NFS clients handle this is to check access via the
Access operation/RPC and use that to determine if an open(2) on the
client is allowed.

It appears that, for NFSv4, the Linux client expects the NFSv4 Open (not a
POSIX open) operation to fail with NFSERR_ACCES if the file is not being
created and file permissions do not allow owner access, unlike NFSv3.
Since both the Linux and OpenSolaris NFSv4 servers seem to exhibit this
behaviour, this patch changes the FreeBSD NFSv4 server to do the same.
A sysctl called vfs.nfsd.v4openaccess can be set to 0 to return the
NFSv4 server to its previous behaviour.

Since both the Linux and FreeBSD NFSv4 clients seem to exhibit correct
behaviour with the access check for file owner in Open enabled, it is enabled
by default.

Reported by:	luoqi.chen@gmail.com
MFC after:	2 weeks