ocfs2: Use cpu_to_le16 for e_leaf_clusters in ocfs2_bg_discontig_add_extent.

e_leaf_clusters is a le16, so use cpu_to_le16 instead
of cpu_to_le32.

What's more, we change 'clusters' to unsigned int to
signify that the size of 'clusters' isn't important here.

Signed-off-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>