btrfs: assert tree is locked when clearing extent map from logging

When calling clear_em_logging() we should have a write lock on the extent
map tree, as we will try to merge the extent map with the previous and
next ones in the tree. So assert that we have a write lock.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>