Fix page fault in zfsctl_snapdir_getattr

Must acquire the z_teardown_lock before accessing the zfsvfs_t object. I
can't reproduce this panic on demand, but this looks like the correct
solution.

PR: 247668
Reviewed by: avg
MFC after: 2 weeks
Sponsored by: Axcient
Differential Revision: https://reviews.freebsd.org/D25543