LU-2675 cleanup: make bitfield (un)signedness explicit

LU-2675 cleanup: make bitfield (un)signedness explicit

Fix this common sparse warning (dubious one-bit signed bitfield) in
case ones-complement machines come back or something.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I24ea468efedd8a1269ea92022d13e802f4838ba1
Reviewed-on: http://review.whamcloud.com/5166
Tested-by: Hudson
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Sebastien Buisson <sebastien.buisson@bull.net>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>