The data update path requires special support for unwritten extents - we
still need to be able to move them, but there's no need to read or write
anything.

This patch adds a new error code to tell bch2_move_extent() that we're
short circuiting the read, and adds bch2_update_unwritten_extent() to
create a reservation then call __bch2_data_update_index_update().
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>