journal: make sure to explicitly copy out values of mmap before doing arithmetics...

journal: make sure to explicitly copy out values of mmap before doing arithmetics on them

Our journal code is generally supposed to be written in a fashion that
the underlying file can be deallocated any time, i.e. our mmap of it
suddenly becomes all zeroes. The idea is that we catch that when parsing
everything. For that to work safely we need to make sure that when doing
arithmetics or comparisons on values read from the map we don't run into
TTOCTTOU issues when determining validity. Hence we need to copy out the
values before use and operate on the copies. This requires some special
care since the C compiler could suppress our copies as optimization.
Hence use the new READ_NOW() macro to force a copy by using memcpy(),
and use it whenever we start doing an arithmetic operation on it, or
validity checking of multiple steps.

Fixes: #14943