zfs_lookup: fix bogus arguments to lookup of "snapshot" directory

When a parent directory lookup is done at the root of a snapshot mounted
under .zfs/snapshot directory, we need to look up that directory in
the parent filesystem. We achieve that by doing a VOP_LOOKUP operation
on a .zfs vnode with "snapshot" as a target name. But previously we
also passed ISDOTDOT flag to the lookup and, because of that, the lookup
actually returned the parent of the .zfs vnode, that is, a root vnode of
the parent filesystem.

Reported by: lev
Tested by: lev
MFC after: 3 days