When the pNFS client is handling mirrored DS servers via the Flexible
File Layout, it must do writes and commits to all the mirrors.
This patch makes all but the last write/commit to a mirror be done
by a set of threads in a taskqueue, so that the RPCs will be done
concurrently.

Since changing the # of threads dynamically can't be done with
taskqueue, the # of threads is a sysctl which defaults to 32.
I think this is ok, since NFS I/O activity is so bursty that previous
I/O activity doesn't allow for good prediction of when threads
will be needed. As such, a fixed # of threads seems reasonable.

I have been testing this code against the pNFS server in
subversion under projects/pnfs-planb-server and it seems
to work ok. I do not know what the overheads of using
a taskqueue is, but I suspect it is less that doing a kproc_create()/kproc_exit()
for every RPC.