btrfs: allocate backref_ctx on stack in find_extent_clone

Instead of using kmalloc() to allocate backref_ctx, allocate backref_ctx
on stack. The size is reasonably small.

sizeof(backref_ctx) = 48

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>