malloc: add alloc_size attribute to functions

malloc: add alloc_size attribute to functions

By using the alloc_size() attribute the compiler can optimize
better and detect errors at compile time.

For example, Gcc will fail one of the invalid allocation examples
in app/test/test_malloc.c because the allocation is outside the
limits of memory.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>