nbd: Silence Coverity false positive

nbd: Silence Coverity false positive

Coverity noticed (CID 1436125) that we check the return value of
nbd_extent_array_add in most places, but not at the end of
bitmap_to_extents().  The return value exists to break loops before a
future iteration, so there is nothing to check if we are already done
iterating.  Adding a cast to void, plus a comment why, pacifies
Coverity.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20201111163510.713855-1-eblake@redhat.com>
[eblake: Prefer cast to void over odd && usage]
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>