idtree: fix handling of large ids (eg INT_MAX)

idtree: fix handling of large ids (eg INT_MAX)

Since idtree assigns sequentially, it rarely reaches high numbers.
But such numbers can be forced with idr_get_new_above(), and that
reveals two bugs:
1) Crash in sub_remove() caused by pa array being too short.
2) Shift by more than 32 in _idr_find(), which is undefined, causing
   the "outside the current tree" optimization to misfire and return NULL.

Signed-off-by: Rusty Russell <rusty@rustorp.com.au>