MFC r248653: ZFS: Fix a panic while unmounting a busy filesystem.

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.