tools/cephfs_mirror: Fix assert while opening handles

tools/cephfs_mirror: Fix assert while opening handles

Issue:
When the crawler or a datasync thread encountered an error,
it's possible that the crawler gets notified by a datasync
thread and bails out resulting in the unregister of the
particular dir_root. The other datasync threads might
still hold the same syncm object and tries to open the
handles during which the following assert is hit.

ceph_assert(it != m_registered.end());

Cause:
This happens because the in_flight counter in syncm object
was tracking if it's processing the actual job from the data
queue.

Fix:
Make in_flight counter in syncm object to track the active
syncm object i.e, inrement as soon as the datasync thread
get a reference to it and decrement when it goes out of
reference.

Fixes: https://tracker.ceph.com/issues/73452
Signed-off-by: Kotresh HR <khiremat@redhat.com>