Revert "x86/ftrace: Use alternative RET encoding"

This reverts commit e54fcb08.

This temporarily reverts the backport of upstream commit
1f001e9d. It was not correct to copy the
ftrace stub as it would contain a relative jump to the return thunk which
would not apply to the context where it was being copied to, leading to
ftrace support to be broken.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>