scripts: kernel-doc: Ignore __alloc_size() attribute

scripts: kernel-doc: Ignore __alloc_size() attribute

Fixes "Compiler Attributes: add __alloc_size() for better bounds checking"
so that the __alloc_size() macro is ignored for function prototypes when
generating kerndoc. Avoids warnings like:

./include/linux/slab.h:662: warning: Function parameter or member '1' not described in '__alloc_size'
./include/linux/slab.h:662: warning: Function parameter or member '2' not described in '__alloc_size'
./include/linux/slab.h:662: warning: expecting prototype for kcalloc().  Prototype was for __alloc_size() instead

Suggested-by: Matthew Wilcox <willy@infradead.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20211011180650.3603988-1-keescook@chromium.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>