ocfs2: set gap to seperate entry and value when xattr in bucket

This patch set a gap (4 bytes) between xattr entry and
name/value when xattr in bucket. This gap use to seperate
entry and name/value when a bucket is full. It had already
been set when xattr in inode/block.

Signed-off-by: Tiger Yang <tiger.yang@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>