ocfs2: make xattr extension work with new local alloc reservation.

The old ocfs2_xattr_extent_allocation is too optimistic about
the clusters we can get. So actually if the file system is
too fragmented, ocfs2_add_clusters_in_btree will return us
with EGAIN and we need to allocate clusters once again.

So this patch change it to a while loop so that we can allocate
clusters until we reach clusters_to_add.

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