Lock around magic_free() in rb_mgc_close() which is Magic#close just in case.

Signed-off-by: Krzysztof Wilczynski <krzysztof.wilczynski@linux.com>
More convenient to have names of flags as strings.

Signed-off-by: Krzysztof Wilczynski <krzysztof.wilczynski@linux.com>