Replace a global map->crush_workspace (protected by a global mutex)
with a list of workspaces, up to the number of CPUs + 1.

This is based on a patch from Robin Geuze <robing@nl.team.blue>.
Robin and his team have observed a 10-20% increase in IOPS on all
queue depths and lower CPU usage as well on a high-end all-NVMe
100GbE cluster.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>