ocfs2: Add an insertion check to ocfs2_extent_tree_operations.

A couple places check an extent_tree for a valid inode.  We move that
out to add an eo_insert_check() operation.  It can be called from
ocfs2_insert_extent() and elsewhere.

We also have the wrapper calls ocfs2_et_insert_check() and
ocfs2_et_sanity_check() ignore NULL ops.  That way we don't have to
provide useless operations for xattr types.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>