dissect: teach do_initializer() to handle the nested EXPR_IDENTIFIER's

dissect: teach do_initializer() to handle the nested EXPR_IDENTIFIER's

do_initializer() is very limited/buggy but it was able to parse the kernel
code until ftrace started to use ".a.b = x" rather than ".a = { .b = x }"
in initializers.

Test-case:

	struct O {
		struct I {
			int mem;
		} inn;
		int end;
	} var = {
		.inn.mem = 0,
		0,
	};

before the patch:

	1:8   s def  O
	2:16  s def  I
	6:3   g def  var                              struct O
	6:3   g -w-  var                              struct O
	7:10  s -w-  O.inn                            struct I
	7:10  s -w-  I.*                              struct I
	I.c:7:14: warning: bad expr->type: 25
	8:9   s -w-  O.end                            int

after:

	1:8   s def  O
	2:16  s def  I
	6:3   g def  var                              struct O
	6:3   g -w-  var                              struct O
	7:10  s -w-  O.inn                            struct I
	7:14  s -w-  I.mem                            int
	8:9   s -w-  O.end                            int

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Lance Richardson <lrichard@redhat.com>
Signed-off-by: Christopher Li <sparse@chrisli.org>