ocfs2: replace u8 by __u8 in ocfs2_fs.h

This patch replaces date type 'u8' with '__u8', which follows the coding style of ocfs2_fs.h, and portable to user space
for ocfs2-tools.

Signed-off-by: Coly Li <coly.li@suse.de>
Signed-off-by: Joel Becker <joel.becker@oracle.com>