[backport] thrust/mr: fix the case of reuising a block for a smaller alloc. (#1232) #1317
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Previously, the pool happily returned a pointer to a larger oversized block than requested, without storing the information that the block is now smaller, which meant that on deallocation, it'd look for the descriptor of the block in the wrong place. This is now fixed by moving the descriptor to always be where deallocation can find it using the user-provided size, and by storing the original size to restore the descriptor to its rightful place when deallocating.
Also a drive-by fix for a bug where in certain cases the reallocated cached oversized block wasn't removed from the cached list. Whoops. Kinda surprised this hasn't exploded before.