rust: warn if `srctree/` links do not exist

rust: warn if `srctree/` links do not exist

`srctree/` links may point to nonexistent files, e.g. due to renames
that missed to update the files or simply because of typos.

Since they can be easily checked for validity, do so and print a warning
in the file does not exist.

This found the following cases already in-tree:

    warning: srctree/ link to include/linux/blk_mq.h does not exist
    warning: srctree/ link to include/linux/drm/drm_gem.h does not exist
    warning: srctree/ link to include/linux/drm/drm_drv.h does not exist
    warning: srctree/ link to include/linux/drm/drm_ioctl.h does not exist
    warning: srctree/ link to include/linux/drm/drm_file.h does not exist
    warning: srctree/ link to include/linux/drm/drm_device.h does not exist

Inspired-by: Onur Özkan <work@onurozkan.dev>
Link: https://lore.kernel.org/rust-for-linux/CANiq72=xCYA7z7_rxpzzKkkhJs6m7L_xEaLMuArVn3ZAcyeHdA@mail.gmail.com/
Reviewed-by: Onur Özkan <work@onurozkan.dev>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Tested-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>