x86/dumpstack: Dump user space code correctly again

H.J. reported that post 5.7 a segfault of a user space task does not longer
dump the Code bytes when /proc/sys/debug/exception-trace is enabled. It
prints 'Code: Bad RIP value.' instead.

This was broken by a recent change which made probe_kernel_read() reject
non-kernel addresses.

Update show_opcodes() so it retrieves user space opcodes via
copy_from_user_nmi().

Fixes: 98a23609

 ("maccess: always use strict semantics for probe_kernel_read")
Reported-by: H.J. Lu <hjl.tools@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/87h7tz306w.fsf@nanos.tec.linutronix.de