dma-debug: switch check from _text to _stext

In include/asm-generic/sections.h:

  /*
   * Usage guidelines:
   * _text, _data: architecture specific, don't use them in
   * arch-independent code
   * [_stext, _etext]: contains .text.* sections, may also contain
   * .rodata.*
   *                   and/or .init.* sections

_text is not guaranteed across architectures.  Architectures such as ARM
may reuse parts which are not actually text and erroneously trigger a bug.
Switch to using _stext which is guaranteed to contain text sections.

Came out of https://lkml.kernel.org/g/<567B1176.4000106@redhat.com>

Signed-off-by: Laura Abbott <labbott@fedoraproject.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/dma-debug.c

@@ -1181,7 +1181,7 @@ static inline bool overlap(void *addr, unsigned long len, void *start, void *end
 
 static void check_for_illegal_area(struct device *dev, void *addr, unsigned long len)
 {
-	if (overlap(addr, len, _text, _etext) ||
+	if (overlap(addr, len, _stext, _etext) ||
 	    overlap(addr, len, __start_rodata, __end_rodata))
 		err_printk(dev, NULL, "DMA-API: device driver maps memory from kernel text or rodata [addr=%p] [len=%lu]\n", addr, len);
 }