GWP-ASan guarded allocation size limitation

Hi!

From what I see in the [implementation](https://github.com/google/tcmalloc/blob/97be09dbeb513355d42c23352f99a02302fac939/tcmalloc/allocation_sampling.h#L98) and in the [documentation](https://google.github.io/tcmalloc/gwp-asan.html#limitations), allocations consisting of more than a single page are not sampled, and therefore are not subject to address sanitizing.

Such limitation contradicts my intuition. Aren't smaller allocations the ones that introduce relatively larger amount of CPU & RAM overhead for guarding? Why is allocation of, let's say, 1 MiB expensive? Doesn't it require only two extra fence pages?