sunrpc: track the max number of requested threads in a pool

sunrpc: track the max number of requested threads in a pool

The kernel currently tracks the number of threads running in a pool in
the "sp_nrthreads" field. In the future, where threads are dynamically
spun up and down, it'll be necessary to keep track of the maximum number
of requested threads separately from the actual number running.

Add a pool->sp_nrthrmax parameter to track this. When userland changes
the number of threads in a pool, update that value accordingly.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>