Update a couple vm_object lock assertions in the swap pager to reflect the
new use of the vm_object's lock to synchronize updates to a radix trie
mapping per-vm object page indices to on-disk swap blocks.

Fix a typo in a nearby comment.

Reviewed by: kib, markj
X-MFC with: r322913
Differential Revision: https://reviews.freebsd.org/D12134