linux/dim: move useful macros to .h file

Useful macros will be used effectively elsewhere.
These will be utilized in subsequent patches.

Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240621101353.107425-2-hengqi@linux.alibaba.com


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