bpf: Correct kfunc argument diagnostics

The Call Type Safety diagnostics mishandle three kfunc argument classes.

BTF type ID 0 represents void, but btf_show_name() also uses zero to end
type traversal. A pointer that resolves to void therefore loses its pointee
name and is rendered as "()". End traversal directly for concrete terminal
types, but resolve referenced types before testing for ID zero, and name the
void terminal type explicitly. Format the complete parameter pointer type
for nullable kfunc arguments, so void pointers are reported as (void *).

Also add the missing structured report when an __szk memory-size argument is
not a verifier-known constant. Describe the generic bpf_refcount_acquire()
contract without deriving an object type from its void pointer prototype.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/668871823f90f69896d3db27b56db2f53e481162.camel@gmail.com
Link: https://lore.kernel.org/bpf/20260816015746.2632990-7-memxor@gmail.com

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