x86/debug: Fix DR6 handling

x86/debug: Fix DR6 handling

Tom reported that one of the GDB test-cases failed, and Boris bisected
it to commit:

  d53d9bc0cf78 ("x86/debug: Change thread.debugreg6 to thread.virtual_dr6")

The debugging session led us to commit:

  6c0aca288e72 ("x86: Ignore trap bits on single step exceptions")

It turns out that TF and data breakpoints are both traps and will be
merged, while instruction breakpoints are faults and will not be merged.
This means 6c0aca288e72 is wrong, only TF and instruction breakpoints
need to be excluded while TF and data breakpoints can be merged.

 [ bp: Massage commit message. ]

Fixes: d53d9bc0cf78 ("x86/debug: Change thread.debugreg6 to thread.virtual_dr6")
Fixes: 6c0aca288e72 ("x86: Ignore trap bits on single step exceptions")
Reported-by: Tom de Vries <tdevries@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/YBMAbQGACujjfz%2Bi@hirez.programming.kicks-ass.net
Link: https://lkml.kernel.org/r/20210128211627.GB4348@worktop.programming.kicks-ass.net