osdc/Objecter: fix linger_ops iterator invalidation on pool deletion

The call to check_linger_pool_dne() may unregister the linger request,
invalidating the iterator.  To avoid this, increment the iterator at
the top of the loop.

This mirror the fix in 4bf9078286d58c2cd4e85cb8b31411220a377092 for
regular non-linger ops.

Fixes: #3734
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>