osd: fix race when queuing recovery ops

Previously we would sample how many ops to start under the lock, drop it,
and start that many.  This is racy because multiple threads can jump in
and we start too many ops.  Instead, claim as many slots as we can and
release them back later if we do not end up using them.

Take care to re-wake the work-queue since we are releasing more resources
for wq use.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>