Backport new field_prep()/get() particularly useful to handle case where a
bitmask is not const and FIELD_PREP can't be used. This permit to replace
manual shift with these macro. (also needed to permit backport of some
patch without modification)
Backport reworked patch that drop the local field_prep()/get() macro in
favor of the global one.
Link: https://github.com/openwrt/openwrt/pull/22479
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
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