selinux: Add __GFP_NOWARN to allocation at str_read()

syzbot is hitting warning at str_read() [1] because len parameter can
become larger than KMALLOC_MAX_SIZE. We don't need to emit warning for
this case.

[1] https://syzkaller.appspot.com/bug?id=7f2f5aad79ea8663c296a2eedb81978401a908f0



Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+ac488b9811036cea7ea0@syzkaller.appspotmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>