9p: Use the i_size_[read, write]() macros instead of using inode->i_size directly.

Change all occurrence of inode->i_size with i_size_read() or i_size_write()
as appropriate.

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