Remove incorrect alloc_size attribute from pvalloc [BZ #25401]

pvalloc is guarantueed to round up the allocation size to the page
size, so applications can assume that the memory region is larger
than the passed-in argument.  The alloc_size attribute cannot express
that.

The test case is based on a suggestion from Jakub Jelinek.

This fixes commit 9bf8e29c ("malloc:
make malloc fail with requests larger than PTRDIFF_MAX (BZ#23741)").

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>