loader: disk_cleanup was left in userboot_disk.c

In r315408, disk_cleanup was removed, which is called at
sys/boot/userboot/userboot/userboot_disk.c:113.
This causes bhyveload to fail.

PR: 217935
Reported by: Fabian Freyer
Reviewed by: allanjude
Approved by: allanjude (mentor)
Differential Revision: https://reviews.freebsd.org/D10060