crimson/common: do not take from a future twice

crimson/common: do not take from a future twice

before this change, in our specialization of seastar::do_until(),
we access `f` after calling `f.get()`, this is not correct. as `f.get()`
actually moves `f._state` away and detaches the associated promise if any.
so we cannot call `f._then()` anymore after calling `f.get()`. as
`f._then()` schedules `f` by detaching the future from promise and
attaching the scheduled task to the promise. but `future_base::detach_promise()`
does not check `_promise` before accessing it, hence the segfault.

after this change, the order of the checks is rearranged so that
`f.get()` is called at the end. and also use `f.get0()` to be more
explicit, as we are accessing the only element of the returned
value.

Signed-off-by: Kefu Chai <kchai@redhat.com>