swap_pager: avoid meta_transfer race

Function swp_pager_meta_transfer expects that after dropping and
reacquiring an object lock, the swap block it's processing still
exists, and has not been removed from the trie and freed. Rewrite to
avoid depending on that, by scrubbing, removing and freeing it before
releasing the lock.

Reviewed by: alc, markj
Differential Revision: https://reviews.freebsd.org/D46629