x86/boot/e820: Call the PCI gap a 'gap' in the boot log printout

It is a bit weird and inconsistent that the PCI gap is
advertised during bootup as 'mem'ory:

  [mem 0xc0000000-0xfed1bfff] available for PCI devices
   ^^^

It's not really memory, it's a gap that PCI devices can decode
and use and they often do not map it to any memory themselves.

So advertise it for what it is, a gap:

  [gap 0xc0000000-0xfed1bfff] available for PCI devices

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Shevchenko <andy@kernel.org>
Cc: Arnd Bergmann <arnd@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Juergen Gross <jgross@suse.com>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Paul Menzel <pmenzel@molgen.mpg.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Link: https://patch.msgid.link/20250515120549.2820541-10-mingo@kernel.org

arch/x86/kernel/e820.c

@@ -699,7 +699,7 @@ __init void e820__setup_pci_gap(void)
 	 */
 	pci_mem_start = gapstart;
 
-	pr_info("[mem %#010lx-%#010lx] available for PCI devices\n",
+	pr_info("[gap %#010lx-%#010lx] available for PCI devices\n",
 		gapstart, gapstart + gapsize - 1);
 }