This particular scenario was easily reproduced using a NFS export. When the
first 'zfs unmount' occurred, it returned EBUSY via this path, while
vflush() had flushed references on the filesystem's root vnode, which in
turn caused its v_interlock to be destroyed. The next time 'zfs unmount'
was called, vflush() tried to obtain this lock, which caused this panic.
Since vflush() on FreeBSD is a definitive call, there is no need to check
vfsp->vfs_count after it completes. Simply #ifdef sun this check.
Submitted by: avg
Reviewed by: avg
Approved by: ken (mentor)
MFC after: 1 month
74c2859109