mgr/volumes: Fix pending_subvolume_deletions in volume info

mgr/volumes: Fix pending_subvolume_deletions in volume info

Problem:
 The pending_subvolume_deletions field in 'fs volume info' output is
always zero

Cause:
 The 'os.listdir' being used to list the trash directory didn't have
subvolume basdir context. Hence it was always failing with ENOENT.

Fix:
 Modify the fs_util's listdir function to list the files as well and use it.

Signed-off-by: Kotresh HR <khiremat@redhat.com>
Fixes: https://tracker.ceph.com/issues/62278
(cherry picked from commit 6ca1bffe57224a84da00a6f714c0535703d45e0d)