Add support for the ModeSetMasked attribute to the NFSv4.1 server.

I do not know of an extant NFSv4.1 client that currently does a Setattr
operation for the ModeSetMasked, but it has been discussed on the linux-nfs
mailing list.
This patch adds support for doing a Setattr of ModeSetMasked, so that it
will work for any future NFSv4.1 client that chooses to do so.
Tested via a hacked FreeBSD NFSv4.1 client.

MFC after: 2 weeks