librbd: workaround an ICE of GCC

librbd: workaround an ICE of GCC

GCC is somehow annoyed at seeing the combination of decltype and
initializer_list in this place. i tried to remove the `if` clause, and
only left the `else` block, GCC was happy with that change. i also tried
to pass an empty `{}` to `decltype(reply.lockers)`, and GCC was also
happy with that. so i guess there are multiple factors taking effect in
this problem. probably any of them could be the last straw that breaks
GCC.

but we cannot have a minimal reproducer for this issue here without more
efforts. and `reply.lockers` is empty after `reply` is constructed, so
it would be simpler if we just add the locker info to it instead of
assigning a newly constructed `map` to it.

Fixes: http://tracker.ceph.com/issues/37719
Signed-off-by: Kefu Chai <kchai@redhat.com>