MFC r324257-r324258

r324257:
mountd: Convert exportlist to SLIST

Use SLIST from sys/queue.h instead of homebrew linked list for the exportlist.

Reviewed by: bapt, rmacklem
Sponsored by: Gandi.net
Differential Revision: https://reviews.freebsd.org/D12502

r324258:
mountd: Convert mountlist to SLIST

Use SLIST from sys/queue.h instead of homebrew linked list for mountlist.

Reviewed by: bapt, rmacklem
Sponsored by: Gandi.net
Differential Revision: https://reviews.freebsd.org/D12504