crush: add unitest for crushtool --show-bad-mappings

Signed-off-by: Loic Dachary <loic@dachary.org>
crush: implement --show-bad-mappings for indep

Support the presence of ITEM_NONE device numbers in the indep mapping as
proof of a bad mapping. Implement the associated unit tests.

Signed-off-by: Loic Dachary <loic@dachary.org>