TOMOYO: Fix incomplete read after seek.

Commit f23571e8

 "TOMOYO: Copy directly to userspace buffer." introduced
tomoyo_flush() that flushes data to be read as soon as possible.
tomoyo_select_domain() (which is called by write()) enqueues data which meant
to be read by next read(), but previous read()'s read buffer's size was not
cleared. As a result, since 2.6.36, sequence like

   char *cp = "select global-pid=1\n";
   read(fd, buf1, sizeof(buf1));
   write(fd, cp, strlen(cp));
   read(fd, buf2, sizeof(buf2));

causes enqueued data to be flushed to buf1 rather than buf2.
Fix this bug by clearing read buffer's size upon write() request.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: James Morris <jmorris@namei.org>