Disable multiple ntfs mounts to the same mountpoint.
Eliminates panics due to locking issues.
Idea taken from src/sys/gnu/fs/xfs/FreeBSD/xfs_super.c.

PR: 89966, 92000, 104393
Reported by: H. Matsuo <hiroshi50000 yahoo co jp>,

		Chris <m2chrischou gmail.com>,
		Andrey V. Elsukov <bu7cher yandex ru>,
		Jan Henrik Sylvester <me janh de>

Approved by: re (kensmith)