vfs_bio: initialize pctries

[dougm]

Summary

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.

[cem]

Lgtm