btrfs: pass NULL index to btrfs_del_inode_ref() where not needed

btrfs: pass NULL index to btrfs_del_inode_ref() where not needed

There are two callers of btrfs_del_inode_ref() that declare a local index
variable and then pass a pointer for it to btrfs_del_inode_ref(), but then
don't use that index at all. Since btrfs_del_inode_ref() accepts a NULL
index pointer, pass NULL instead and stop declaring those useless index
variables.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>