ocfs2: Abstract ocfs2 xattr tree extend rec iteration process.

Currently we have ocfs2_iterate_xattr_buckets which can receive
a para and a callback to iterate a series of bucket. It is good.
But actually the 2 callers ocfs2_xattr_tree_list_index_block and
ocfs2_delete_xattr_index_block are almost the same. The only
difference is that the latter need to handle the extent record
also. So add a new function named ocfs2_iterate_xattr_index_block.
It can be given func callback which are used for exten record.
So now we only have one iteration function for the xattr index
block. Ane what's more, it is useful for our future reflink
operations.

Signed-off-by: Tao Ma <tao.ma@oracle.com>