x86/ptrace: Remove set_segment_reg() implementations for current

seg_segment_reg() should be unreachable with task == current.
Rather than confusingly trying to make it work, just explicitly
disable this case.

(regset->get is used for current in the coredump code, but the ->set
 interface is only used for ptrace, and you can't ptrace yourself.)

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>