NFS: Fix an Oopsable condition in __nfs_pageio_add_request()

NFS: Fix an Oopsable condition in __nfs_pageio_add_request()

commit 56517ab958b7c11030e626250c00b9b1a24b41eb upstream.

Ensure that nfs_pageio_error_cleanup() resets the mirror array contents,
so that the structure reflects the fact that it is now empty.
Also change the test in nfs_pageio_do_add_request() to be more robust by
checking whether or not the list is empty rather than relying on the
value of pg_count.

Fixes: a7d42ddb3099 ("nfs: add mirroring support to pgio layer")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>