NFSv4.1: Do not call pnfs_return_layout() from an rpciod context

Move the call to pnfs_return_layout() to the read and write rpc_release()
callbacks, so that it gets called from nfsiod, which is a more appropriate
context.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>