NFS: Fix use-after-free issues in nfs_pageio_add_request()

We need to ensure that we create the mirror requests before calling
nfs_pageio_add_request_mirror() on the request we are adding.
Otherwise, we can end up with a use-after-free if the call to
nfs_pageio_add_request_mirror() triggers I/O.

Fixes: c917cfaf ("NFS: Fix up NFS I/O subrequest creation")
Cc: stable@vger.kernel.org
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>