safesetid: check size of policy writes

syzbot attempts to write a buffer with a large size to a sysfs entry
with writes handled by handle_policy_update(), triggering a warning
in kmalloc.

Check the size specified for write buffers before allocating.

Reported-by:  <syzbot+4eb7a741b3216020043a@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=4eb7a741b3216020043a


Signed-off-by: Leo Stone <leocstone@gmail.com>
[PM: subject tweak]
Signed-off-by: Paul Moore <paul@paul-moore.com>