use __u32 in linux/reiserfs_fs.h

Since this header is exported to userspace and all the other types in the
header have been scrubbed, this brings the last straggler in line.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>