crimson/scrub: fix assert in PGScrubber::release_range() on interval change

crimson/scrub: fix assert in PGScrubber::release_range() on interval change

when an interval change occurs while ScrubReserveRange is still
waiting to acquire background_process_lock, ChunkState::exit()
calls release_range() but blocked is not yet set. this triggers
ceph_assert(blocked) in release_range().

fix by checking if blocked is set before asserting. if blocked is
not set, the range was never reserved, so release_range() is a
no-op. ScrubReserveRange's finally block handles lock cleanup in
this case.

Fixes: https://tracker.ceph.com/issues/76752
Signed-off-by: Kefu Chai <k.chai@proxmox.com>