dump_iterate() dumps blocks in a loop.  Eventually, get_next_block()
returns "no more".  We then call dump_completed().  But we neglect to
break the loop!  Broken in commit 4c7e251a.

Because of that, we dump the last block again.  This attempts to write
to s->fd, which fails if we're lucky.  The error makes dump_iterate()
return failure.  It's the only way it can ever return.

Theoretical: if we're not so lucky, something else has opened something
for writing and got the same fd.  dump_iterate() then keeps looping,
messing up the something else's output, until a write fails, or the
process mercifully terminates.

The obvious fix is to restore the return lost in commit 4c7e251a

.  But
the root cause of the bug is needlessly opaque loop control.  Replace it
by a clean do ... while loop.

This makes the badly chosen return values of get_next_block() more
visible.  Cleaning that up is outside the scope of this bug fix.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>