os/bluestore: fix incorrect bluefs_used accounting when releasing old extents

os/bluestore: fix incorrect bluefs_used accounting when releasing old extents

`bluefs_used` was decremented using to_release.size(), which represents the number
of extents (always 1 in this case), instead of the actual extent length in bytes.
This resulted in incorrect BlueFS space accounting when using the shared allocator.

The fix updates the decrement to use `old_ext.length`, ensuring accurate tracking.

Fixes: https://tracker.ceph.com/issues/75028
Signed-off-by: Jaya Prakash <jayaprakash@ibm.com>