Revert "x86: Pin task-stack in __get_wchan()"

This reverts commit b62c8ee4 which is
commit 0dc636b3 upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: I382908f0d8a1fc0e3ea64cd51fd835aeee520cd5
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>