As of 9e6544dd6e contigfree(9) is no longer
needed and should not be used anymore. We leave a wrapper for 3rd party
code in at least 15.x but remove (almost) all other cases from the tree.
This leaves one use of contigfree(9) untouched; that was the original
trigger for 9e6544dd6e and is handled in D45813 (to be committed
seperately later).
Sponsored by: The FreeBSD Foundation
Reviewed by: markj, kib
Tested by: pho (10h stress test run)
Differential Revision: https://reviews.freebsd.org/D46099
d1bdc2821f