core:fs: Use a much better union structure in file structure

core:fs: Use a much better union structure in file structure

Remove the u1 and u2 union, and merge the related fileds as a group, and
then make a big union. Then we don't need to use u1 or u2 to reference
these fileds any more but use it directly just like there are no union
structures here:)

Signed-off-by: Liu Aleaxander <Aleaxander@gmail.com>