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I often regenerate all YNL files in the tree to make sure they
are in sync with the codegen and specs. Generator rewrites
the files unconditionally, so since make looks at file modification
time to decide what to rebuild - my next build takes longer.

We already generate the code to a tempfile most of the time,
only overwrite the target when we have to.

Before:

  $ stat include/uapi/linux/netdev.h
    File: include/uapi/linux/netdev.h
    Size: 2307      	Blocks: 8          IO Block: 4096   regular file
  Access: 2023-10-27 15:19:56.347071940 -0700
  Modify: 2023-10-27 15:19:45.089000900 -0700
  Change: 2023-10-27 15:19:45.089000900 -0700
   Birth: 2023-10-27 15:19:45.088000894 -0700

  $ ./tools/net/ynl/ynl-regen.sh -f
  [...]

  $ stat include/uapi/linux/netdev.h
    File: include/uapi/linux/netdev.h
    Size: 2307      	Blocks: 8          IO Block: 4096   regular file
  Access: 2023-10-27 15:19:56.347071940 -0700
  Modify: 2023-10-27 15:22:18.417968446 -0700
  Change: 2023-10-27 15:22:18.417968446 -0700
   Birth: 2023-10-27 15:19:45.088000894 -0700

After:

  $ stat include/uapi/linux/netdev.h
    File: include/uapi/linux/netdev.h
    Size: 2307      	Blocks: 8          IO Block: 4096   regular file
  Access: 2023-10-27 15:22:41.520114221 -0700
  Modify: 2023-10-27 15:22:18.417968446 -0700
  Change: 2023-10-27 15:22:18.417968446 -0700
   Birth: 2023-10-27 15:19:45.088000894 -0700

  $ ./tools/net/ynl/ynl-regen.sh -f
  [...]

  $ stat include/uapi/linux/netdev.h
    File: include/uapi/linux/netdev.h
    Size: 2307      	Blocks: 8          IO Block: 4096   regular file
  Access: 2023-10-27 15:22:41.520114221 -0700
  Modify: 2023-10-27 15:22:18.417968446 -0700
  Change: 2023-10-27 15:22:18.417968446 -0700
   Birth: 2023-10-27 15:19:45.088000894 -0700
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20231027223408.1865704-1-kuba@kernel.org

Signed-off-by: Jakub Kicinski <kuba@kernel.org>