patch that replaces a single linked list with a hash table of lists (mountd.c) for review

Hi,

I just put a patch for mountd.c in phabricator as D20270, which replaces the
single linked list of structures for exported file systems with a hash table of lists.
This is part of what I hope will fix the performance of mountd when reloading
the exports file(s) for a server with a lot of exported file systems.
Peter Eriksson has reported that his file server with 72000+ exported file systems
takes 16sec to reload the exports file(s), which implies that the nfsd threads are
suspended for 16sec whenever this happens.

If anyone is willing to review this patch, please do so.

Thanks, rick