tools: ynl-gen: move the count into a presence struct too

tools: ynl-gen: move the count into a presence struct too

While we reshuffle the presence members, move the counts as well.
Previously array count members would have been place directly in
the struct, so:

  struct family_op_req {
      struct {
            u32 a:1;
            u32 b:1;
      } _present;
      struct {
            u32 bin;
      } _len;

      u32 a;
      u64 b;
      const unsigned char *bin;
      u32 n_multi;                 << count
      u32 *multi;                  << objects
  };

Since len has been moved to its own presence struct move the count
as well:

  struct family_op_req {
      struct {
            u32 a:1;
            u32 b:1;
      } _present;
      struct {
            u32 bin;
      } _len;
      struct {
            u32 multi;             << count
      } _count;

      u32 a;
      u64 b;
      const unsigned char *bin;
      u32 *multi;                  << objects
  };

This improves the consistency and allows us to remove some hacks
in the codegen. Unlike for len there is no known name collision
with the existing scheme.

Link: https://patch.msgid.link/20250505165208.248049-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>