CVE-2026-98267: 9p: Fix v9fs_issue_write() to update i_size and remote_i_size

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

9p: Fix v9fsissuewrite() to update isize and remoteisize

Fix v9fsissuewrite() to update isize and remoteisize to the new size of the server file if we made it larger, using the start fpos and the count returned by p9clientwrite() to calculate the new minimum file size.

This assumes that if the 9P server makes a short write (say it hits ENOSPC), a reduced count is returned.