mm: move is_kernel_rodata() to asm-generic/sections.h

Export this routine so that we can use it later in devm_kstrdup_const()
and devm_kfree().

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Acked-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Acked-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>