pipe: Fix pipe_full() test in opipe_prep().

syzbot is reporting that splice()ing from non-empty read side to
already-full write side causes unkillable task, for opipe_prep() is by
error not inverting pipe_full() test.

  CPU: 0 PID: 9460 Comm: syz-executor.5 Not tainted 5.6.0-rc3-next-20200228-syzkaller #0
  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
  RIP: 0010:rol32 include/linux/bitops.h:105 [inline]
  RIP: 0010:iterate_chain_key kernel/locking/lockdep.c:369 [inline]
  RIP: 0010:__lock_acquire+0x6a3/0x5270 kernel/locking/lockdep.c:4178
  Call Trace:
     lock_acquire+0x197/0x420 kernel/locking/lockdep.c:4720
     __mutex_lock_common kernel/locking/mutex.c:956 [inline]
     __mutex_lock+0x156/0x13c0 kernel/locking/mutex.c:1103
     pipe_lock_nested fs/pipe.c:66 [inline]
     pipe_double_lock+0x1a0/0x1e0 fs/pipe.c:104
     splice_pipe_to_pipe fs/splice.c:1562 [inline]
     do_splice+0x35f/0x1520 fs/splice.c:1141
     __do_sys_splice fs/splice.c:1447 [inline]
     __se_sys_splice fs/splice.c:1427 [inline]
     __x64_sys_splice+0x2b5/0x320 fs/splice.c:1427
     do_syscall_64+0xf6/0x790 arch/x86/entry/common.c:295
     entry_SYSCALL_64_after_hwframe+0x49/0xbe

Reported-by:  <syzbot+b48daca8639150bc5e73@syzkaller.appspotmail.com>
Link: https://syzkaller.appspot.com/bug?id=9386d051e11e09973d5a4cf79af5e8cedf79386d


Fixes: 8cefc107 ("pipe: Use head and tail pointers for the ring, not cursor and length")
Cc: stable@vger.kernel.org # 5.5+
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>