osd: tolerate racing threads starting recovery ops

We sample the (max - active) recovery ops to know how many to start, but
do not hold the lock over the full duration, such that it is possible to
start too many ops.  This isn't problematic except that our condition
checks for being == max but not beyond it, and we will continue to start
recovery ops when we shouldn't.  Fix this by adjusting the conditional
to be <=.

Reported-by: Stefan Priebe <s.priebe@profihost.ag>
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: David Zafman <david.zafman@inktank.com>