zfsctl: fix several problems with reference counts

• Remove excessive references on a snapshot mountpoint vnode. zfsctl_snapdir_lookup() called VN_HOLD() on a vnode returned from zfsctl_snapshot_mknode() and the latter also had a call to VN_HOLD() on the same vnode. On top of that gfs_dir_create() already returns the vnode with the use count of 1 (set in getnewvnode). So there was 3 references on the vnode.

• mount_snapshot() should keep a reference to a covered vnode. That reference is owned by the mountpoint (mounted snapshot filesystem).

• Remove cryptic manipulations of a covered vnode in zfs_umount(). FreeBSD dounmount() already does the right thing and releases the covered vnode.

PR: 207464
Reported by: dustinwenz@ebureau.com
Tested by: Howard Powell <hpowell@lighthouseinstruments.com>
MFC after: 3 weeks