ocfs2: Use struct ocfs2_extent_tree in ocfs2_num_free_extents().

ocfs2_num_free_extents() re-implements the logic of
ocfs2_get_extent_tree().  Now that ocfs2_get_extent_tree() does not
allocate, let's use it in ocfs2_num_free_extents() to simplify the code.

The inode validation code in ocfs2_num_free_extents() is not needed.
All callers are passing in pre-validated inodes.

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