Btrfs: reorder tree mod log operations in deleting a pointer

Since we don't use MOD_LOG_KEY_REMOVE_WHILE_MOVING to add nritems
during rewinding, we should insert a MOD_LOG_KEY_REMOVE operation first.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>