Support struct arguments and return values in kmodify

[osandov]

Struct/union arguments (passed by value) and struct/union return values have a special calling convention. drgn.helpers.experimental.kmodify.call_function() currently doesn't support either:

drgn/drgn/helpers/experimental/kmodify.py

Lines 1146 to 1153 in 2150a90

	if return_type.kind not in {
	TypeKind.VOID,
	TypeKind.INT,
	TypeKind.BOOL,
	TypeKind.ENUM,
	TypeKind.POINTER,
	}:
	raise NotImplementedError(f"{return_type} return values not implemented")

drgn/drgn/helpers/experimental/kmodify.py

Lines 1251 to 1259 in 2150a90

	if type.kind in {
	TypeKind.FLOAT,
	TypeKind.STRUCT,
	TypeKind.UNION,
	TypeKind.CLASS,
	}:
	raise NotImplementedError(
	f"passing {type} by value not implemented"
	)

It's not super common to pass or return structs in the kernel, but it does happen, so it'd be good to support it. The calling convention on x86-64 is something like small structs get split up and passed/returned in registers, and large structs are put on the stack. The psABI has the details.

We'll definitely want tests for all of the cases in tests/linux_kernel/helpers/test_kmodify.py using tests/linux_kernel/kmod/drgn_test.c.