9p: Fix the incorrect update of inode size in v9fs_file_write()

When using the cache=loose flags, the inode's size was not being
updated correctly on a remote write. Thus subsequent reads of
the whole file resulted in a truncated read. Fix it.

Signed-off-by: Abhishek Kulkarni <adkulkar@umail.iu.edu>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>