Crush: Ensuring at most num-rep osds are selected

Crush temporary buffers are allocated as per replica size configured
by the user.When there are more final osds (to be selected as per
rule) than the replicas, buffer overlaps and it causes crash.Now, it
ensures that at most num-rep osds are selected even if more number of
osds are allowed by the rule.

Fixes: #9492

Signed-off-by: Johnu George <johnugeo@cisco.com>