rgw: address crash and race in RGWIndexCompletionManager

rgw: address crash and race in RGWIndexCompletionManager

An atomic int was used in a modulo operator to distribute contention
among a set of locks and to track completions. Because it was an int,
enough increments would cause it to go negative (due to
twos-complement encoding and overflow) thereby causing a
crash. Additionally, even though it was atomic, the read and increment
were separate operations, leading to a race.

This commit addresses both of these issues.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>
(cherry picked from commit 41f4e83cf5c1c89cd2aa22658716239c4197db0e)