rbd: fix #4060 show data-pool usage when configured

rbd: fix #4060 show data-pool usage when configured

When a data-pool is configured, use it for status infos. The 'data-pool'
config option is used to mark the erasure coded pool while the 'pool'
will be the replicated pool holding meta data such as the omap.

This means, the 'pool' will only use a small amount of space and people
are interested how much they can store in the erasure coded pool anyway.

Therefore this patch reorders the assignment of the used pool name by
availability of the scfg parameters: data-pool -> pool -> fallback 'rbd'

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>