tomoyo: fix UAF write bug in tomoyo_write_control()

In the Linux kernel, the following vulnerability has been resolved:

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.