[REGRESSION 5.8] x86/entry: DR0 break-on-write not working

[Kyle Huey]

rr, a userspace record and replay debugger[0], has a test suite that
attempts to exercise strange corners of the Linux API. One such
test[1] began failing after 2bbc68f8373c0631ebf137f376fbea00e8086be7.
I have not tried to understand what has changed in the kernel here but
since the commit message says "No functional change" I assume
something has gone wrong.

The test expects to get a SIGTRAP when watchvar is written to in the
forked child, but instead the program just exits normally and we get a
status value corresponding to that (exit code 77 = wait status
0x4d00). This test program should be usable outside of rr's test suite
if you replace the test_assert/atomic_puts functions with
assert/printf and replace the util.h include with appropriate standard
includes.

This regression is present in 5.8.

- Kyle

[0] https://rr-project.org/
[1] https://github.com/mozilla/rr/blob/master/src/test/x86/ptrace_debug_regs.c#L55