Cross-shard realloc shrink crashes

[travisdowns]

If a pointer is allocated on one shard and then reallocated smaller on another it crashes in most cases [1], e.g., this test crashes:

SEASTAR_TEST_CASE(test_cross_thread_realloc) {
    // needs at least 2 shards to test what we want to test but will
    // still pass fine on 1 shard

    void *p = malloc(100);

    auto other_shard = (this_shard_id() + 1) % smp::count;

    return smp::submit_to(other_shard, [p]{
        BOOST_REQUIRE(realloc(p, 50));
    });
}

Crash:

alloc_test: /home/tdowns/dev/seastar/src/core/memory.cc:1173: void seastar::memory::cpu_pages::shrink(void *, size_t): Assertion `object_cpu_id(ptr) == cpu_id' failed.

This seems to violate the general contract of realloc and I think should be handled in the same way as cross-core frees. Fix incoming.

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[1] The exceptions are if the new size is zero or the size class of the new size is the same as the old.