For pNFS servers that specify that Layouts are to be returned
upon close, they may expect that LayoutReturn to happen before
the associated Close.

This patch modifies the NFSv4.1/4.2 pNFS client so that this
is done.  This only affects a pNFS mount against a non-FreeBSD
NFSv4.1/4.2 server that specifies return_on_close in LayoutGet
replies.

Found during a recent IETF NFSv4 working group testing event.

MFC after:	2 weeks