ppp: use vmalloc_array() to simplify code

Remove array_size() calls and replace vmalloc() with vmalloc_array() in
bsd_alloc().

vmalloc_array() is also optimized better, resulting in less instructions
being used.

Signed-off-by: Qianfeng Rong <rongqianfeng@vivo.com>
Link: https://patch.msgid.link/20250816090659.117699-4-rongqianfeng@vivo.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>