tools: ynl-gen: refactor local vars for .attr_put() callers

tools: ynl-gen: refactor local vars for .attr_put() callers

Refactor the generation of local variables needed when building
requests, by moving the logic from put_req_nested() into a new
helper put_local_vars(), and use the helper before .attr_put() is
called, thus generating the local variables assumed by .attr_put().

Previously only put_req_nested() generated the variables assumed
by .attr_put(),  print_req() only generated the count iterator `i`,
and print_dump() neither generated `i` nor `array`.

This patch fixes the build errors below:
$ make -C tools/net/ynl/generated/
[...]
-e      GEN wireguard-user.c
-e      GEN wireguard-user.h
-e      CC wireguard-user.o
wireguard-user.c: In function ‘wireguard_get_device_dump’:
wireguard-user.c:480:9: error: ‘array’ undeclared (first use in func)
  480 |         array = ynl_attr_nest_start(nlh, WGDEVICE_A_PEERS);
      |         ^~~~~
wireguard-user.c:480:9: note: each undeclared identifier is reported
                        only once for each function it appears in
wireguard-user.c:481:14: error: ‘i’ undeclared (first use in func)
  481 |         for (i = 0; i < req->_count.peers; i++)
      |              ^
wireguard-user.c: In function ‘wireguard_set_device’:
wireguard-user.c:533:9: error: ‘array’ undeclared (first use in func)
  533 |         array = ynl_attr_nest_start(nlh, WGDEVICE_A_PEERS);
      |         ^~~~~
make: *** [Makefile:52: wireguard-user.o] Error 1
make: Leaving directory '/usr/src/linux/tools/net/ynl/generated'

Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250915144301.725949-5-ast@fiberby.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>