rbd_support: fix perf iostat pool-spec to filter by data pool

The pool-spec filter for 'rbd perf image iostat' and 'rbd perf image
iotop' now correctly refers to the pool where image data is stored.
Previously, the MGR rbd_support module extracted the metadata pool id
from the object name and used it for filtering, which caused EC images
(whose data lives in a separate pool) to be silently dropped when
filtering by pool.

The fix separates the data pool id (k[0][0], where I/O lands) from the
metadata pool id (k[2][0], encoded in the object name for cross-pool
images). Filtering is done against the data pool, while the metadata
pool is preserved for image name resolution and display.

Fixes: https://tracker.ceph.com/issues/75480
Signed-off-by: Dongdong Tao <dongdong.tao@canonical.com>