crush/mapper: finish adding choose_local_[fallback_]tries

Didn't finish this in d129e09e57fbc61cfd4f492e3ee77d0750c9d292.

Signed-off-by: Sage Weil <sage@inktank.com>
crush/mapper: attempts -> tries

Signed-off-by: Sage Weil <sage@inktank.com>
crush/mapper: fix crush_choose_firstn comment

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #968 from ceph/wip-crush

make check is happy and the change is non controversial

Reviewed-by: Loic Dachary <loic@dachary.org>