NFS: Fix an Oopsable condition in __nfs_pageio_add_request()

In the Linux kernel, the following vulnerability has been resolved:

NFS: Fix an Oopsable condition in __nfs_pageio_add_request()

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.