Fix incomplete __mntput locking

Getting this wrong caused

	WARNING: at fs/namespace.c:636 mntput_no_expire+0xac/0xf2()

due to optimistically checking cpu_writer->mnt outside the spinlock.

Here's what we really want:
 * we know that nobody will set cpu_writer->mnt to mnt from now on
 * all changes to that sucker are done under cpu_writer->lock
 * we want the laziest equivalent of
	spin_lock(&cpu_writer->lock);
	if (likely(cpu_writer->mnt != mnt)) {
		spin_unlock(&cpu_writer->lock);
		continue;
	}
	/* do stuff */
  that would make sure we won't miss earlier setting of ->mnt done by
  another CPU.

Anyway, for now we just move the spin_lock() earlier and move the test
into the properly locked region.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Reported-and-tested-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>