MFC r267362:
Remove unneeded mountlist_mtx acquisition from sync_fsync().

All struct mount fields accessed by sync_fsync() are protected by MNT_MTX.
MFC r267392:
Implement simple direct-mapped cache for popular filesystem identifiers to
avoid congestion on global mountlist_mtx mutex in vfs_busyfs(), while
traversing through the list of mount points.

This change significantly improves NFS server scalability, since it had
to do this translation for every request, and the global lock becomes quite
congested.

This code is more optimized for relatively small number of mount points.
On systems with hundreds of active mount points this simple cache may have
many collisions.  But the original traversal code in that case should also
behave much worse, so we are not loosing much.
MFC r267387:
Allocating new bucket for bucket zone, never take it from the zone itself,
since it will almost certanly fail.  Take next bigger zone instead.

This situation should not happen with original bucket zones configuration:
"32 Bucket" zone uses "64 Bucket" and vice versa.  But if "64 Bucket" zone
lock is congested, zone may grow its bucket size and start biting itself.
MFC r267391:
Introduce new "256 Bucket" zone to split requests and reduce congestion
on "128 Bucket" zone lock.