vfs_bio: initialize pctries

bufobj_init depends on fields bo_dirty.bv_root and bo_clean.bv_root
being zeroed on entry and pctrie_init zeroing whatever is passed to
them, and so does not call pctrie_init for either of them. That fails
if pctrie_init ever changes to do something other that zeroing data,
so add explicit calls to them.

Reviewed by: cem
Differential Revision: https://reviews.freebsd.org/D40978