test_rgw_admin_meta: fix warnings

test_rgw_admin_meta: fix warnings

test/test_rgw_admin_meta.cc: In member function 'int admin_meta::test_helper::extract_input(int, char**)':
warning: test/test_rgw_admin_meta.cc:126:24: comparison between signed and unsigned integer expressions [-Wsign-compare]
warning: test/test_rgw_admin_meta.cc:128:24: comparison between signed and unsigned integer expressions [-Wsign-compare]
warning: test/test_rgw_admin_meta.cc:130:24: comparison between signed and unsigned integer expressions [-Wsign-compare]
warning: test/test_rgw_admin_meta.cc:132:24: comparison between signed and unsigned integer expressions [-Wsign-compare]

on arm7l

Signed-off-by: Sage Weil <sage@inktank.com>