fix .zfs-related cases in zfs_lookup that were broken by r303763

The problem is that the special .zfs nodes are not represented by
znodes but by special gfs-based nodes.
r303763 changed interface of zfs_dirlook such that started operating on
znodes rather than on vnodes and, thus, the function became unsuitable
for handling .zfs entities.
The solution is to move the handling of the special cases to zfs_lookup,
the only consumer of zfs_dirlook.
I already had this solution applied in D7421, but for different reasons.

Reported by: asomers
MFC after: 3 days
X-MFC with: r303763