Fix huge_palloc() regression.

Split arena_choose() into arena_[i]choose() and use arena_ichoose() for
arena lookup during internal allocation.  This fixes huge_palloc() so
that it always succeeds during extent node allocation.

This regression was introduced by
66cd953514a18477eb49732e40d5c2ab5f1b12c5 (Do not allocate metadata via
non-auto arenas, nor tcaches.).
9 files changed