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Add rules to ignore data-racy reads with only 1-bit value changes.
Details about the rules are captured in comments in
kernel/kcsan/permissive.h. More background follows.

While investigating a number of data races, we've encountered data-racy
accesses on flags variables to be very common. The typical pattern is a
reader masking all but one bit, and/or the writer setting/clearing only
1 bit (current->flags being a frequently encountered case; more examples
in mm/sl[au]b.c, which disable KCSAN for this reason).

Since these types of data-racy accesses are common (with the assumption
they are intentional and hard to miscompile) having the option (with
CONFIG_KCSAN_PERMISSIVE=y) to filter them will avoid forcing everyone to
mark them, and deliberately left to preference at this time.

One important motivation for having this option built-in is to move
closer to being able to enable KCSAN on CI systems or for testers
wishing to test the whole kernel, whi...