tomoyo: fix UAF write bug in tomoyo_write_control()

Since tomoyo_write_control() updates head->write_buf when write()
of long lines is requested, we need to fetch head->write_buf after
head->io_sem is held.  Otherwise, concurrent write() requests can
cause use-after-free-write and double-free problems.

Reported-by: Sam Sun <samsun1006219@gmail.com>
Closes: https://lkml.kernel.org/r/CAEkJfYNDspuGxYx5kym8Lvp--D36CMDUErg4rxfWFJuPbbji8g@mail.gmail.com
Fixes: bd03a3e4

 ("TOMOYO: Add policy namespace support.")
Cc:  <stable@vger.kernel.org> # Linux 3.1+
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>