9p: Pass the correct end of buffer to p9dirent_read

A patch was accepted recently for sending correct buffer size to p9stat_read.
We need a similar patch in v9fs_dir_readdir_dotl to send correct end of buffer
to p9dirent_read.

Signed-off-by: Sripathi Kodi <sripathik@in.ibm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>