nfscl: Do pNFS layout return_on_close synchronously

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