bitfields should be unsigned.

This fixes sparse noise:
  error: dubious one-bit signed bitfield
Signed-off-by: NH Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Jan Kara <jack@ucw.cz>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>