MFC r321794: Improve FHA locality control for NFS read/write requests.

This change adds two new tunables, allowing to control serialization for
read and write NFS requests separately. It does not change the default
behavior since there are too many factors to consider, but gives additional
space for further experiments and tuning.

The main motivation for this change is very low write speed in case of ZFS
with sync=always or when NFS clients requests sychronous operation, when
every separate request has to be written/flushed to ZIL, and requests are
processed one at a time. Setting vfs.nfsd.fha.write=0 in that case allows
to increase ZIL throughput by several times by coalescing writes and cache
flushes. There is a worry that doing it may increase data fragmentation
on disks, but I suppose it should not happen for pool with SLOG.

Sponsored by: iXsystems, Inc.

MFC r260097:
Move most of NFS file handle affinity code out of the heavily congested
global RPC thread pool lock and protect it with own set of locks.

On synthetic benchmarks this improves peak NFS request rate by 40%.

MFC r259765:
Fix RPC server threads file handle affinity to work better with ZFS.

Instead of taking 8 specific bytes of file handle to identify file during
RPC thread affitinity handling, use trivial hash of the full file handle.
ZFS's struct zfid_short does not have padding field after the length field,
as result, originally picked 8 bytes are loosing lower 16 bits of object ID,
causing many false matches and unneeded requests affinity to same thread.
This fix substantially improves NFS server latency and scalability in SPEC
NFS benchmark by more flexible use of multiple NFS threads.

MFC r260097:
Move most of NFS file handle affinity code out of the heavily congested
global RPC thread pool lock and protect it with own set of locks.

On synthetic benchmarks this improves peak NFS request rate by 40%.

MFC r259765:
Fix RPC server threads file handle affinity to work better with ZFS.

Instead of taking 8 specific bytes of file handle to identify file during
RPC thread affitinity handling, use trivial hash of the full file handle.
ZFS's struct zfid_short does not have padding field after the length field,
as result, originally picked 8 bytes are loosing lower 16 bits of object ID,
causing many false matches and unneeded requests affinity to same thread.
This fix substantially improves NFS server latency and scalability in SPEC
NFS benchmark by more flexible use of multiple NFS threads.