ceph-filestore-dump: fix warnings on i386 build

tools/ceph-filestore-dump.cc: In member function ‘int header::get_header()’:
warning: tools/ceph-filestore-dump.cc:454:19: comparison between signed and unsigned integer expressions [-Wsign-compare]
tools/ceph-filestore-dump.cc: In member function ‘int footer::get_footer()’:
warning: tools/ceph-filestore-dump.cc:471:19: comparison between signed and unsigned integer expressions [-Wsign-compare]
tools/ceph-filestore-dump.cc: In member function ‘int super_header::read_super()’:
warning: tools/ceph-filestore-dump.cc:697:30: comparison between signed and unsigned integer expressions [-Wsign-compare]

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