MFC r328237:

MFC r328237:
Use the __alloc_size2 attribute where relevant.

This follows the documented use in GCC. It is basically only relevant for
calloc(3), reallocarray(3) and  mallocarray(9).

NOTE: Without this change clang 5.0.1 can produce incorrect optimisation
code for static processing of data using the allocated object. For example
this has been seen compiling the mlx4 core module, which allocates a
fixed size array which is then sorted by a fixed order loop. The
optimised result, -O2, is incorrect unless this patch is in place.

Suggested by:	Mark Millard
Reference:	https://docs.freebsd.org/cgi/mid.cgi?9DE674C6-EAA3-4E8A-906F-446E74D82FC4