crimson/osd: mark ThrottleReleaser [[nodiscard]]

crimson/osd: mark ThrottleReleaser [[nodiscard]]

OperationThrottler::get_throttle() returns a ThrottleReleaser guard whose
destructor calls release_throttle(), the mClock RequestCompletion that
decrements in_progress.  The guard has to be held for the whole operation: a
caller that discards it at acquisition releases the slot in the same step, so
the operation never counts against max_in_progress and the throttle does
nothing.

Mark the type [[nodiscard]] so the compiler flags a discarded co_await of
get_throttle().

Signed-off-by: Kefu Chai <k.chai@proxmox.com>