btrfs: assert extent map is not in a list when setting it up

When setting up a new extent map, at setup_extent_mapping(), we're doing
a list move operation to add the extent map the tree's list of modified
extents. This is confusing because at this point the extent map can not
be in any list, because it's a new extent map. So replace the list move
with a list add and add an assertion that checks that the extent map is
not currently in any list.

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