pNFS: Fix races between return-on-close and layoutreturn.

If one or more of the layout segments reports an error during I/O, then
we may have to send a layoutreturn to report the error back to the NFS
metadata server.
This patch ensures that the return-on-close code can detect the
outstanding layoutreturn, and not preempt it.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>