linux-imx

btrfs: assert extent map tree lock in add_extent_mapping

As add_extent_mapping is called from several functions, let's add the
lock annotation. The tree is going to be modified so it must be the
exclusive lock.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>