mon: detect existing fs and duplicate name earlier

mon: detect existing fs and duplicate name earlier

Detecting creation of an equivalent fs was made impossible by
052c3d3f681017d56b5b0ee5cf6f65bffc952a4c, as an existing file system
will obviously have objects in its pool. This commit moves the pool with
objects check later and uses a more helpful errno (EEXIST) for the case
where a fs with the same name already exists (but uses different pools).

Fixes: http://tracker.ceph.com/issues/18964
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>