fs-writeback.c: bitfields should be unsigned

This fixes sparse noise:
  error: dubious one-bit signed bitfield

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>