x86/asm: Annotate movdir64b()'s dst argument with __iomem

Add a missing __iomem annotation to address a sparse warning. The caller
is expected to pass an __iomem annotated pointer to this function. The
current usages send a 64-bytes command descriptor to an MMIO location
(portal) on a device for consumption. When future usages for the
MOVDIR64B instruction warrant a separate variant of a memory to memory
operation, the argument annotation can be revisited.

Also, from the comment in movdir64b() @__dst must be supplied as an
lvalue because this tells the compiler what the object is (its size) the
instruction accesses. I.e., not the pointers but what they point to,
thus the deref'ing '*'."

The actual sparse warning is:

  sparse warnings: (new ones prefixed by >>)
     drivers/dma/idxd/submit.c: note: in included file (through include/linux/io.h, include/linux/pci.h):
  >> arch/x86/include/asm/io.h:422:27: sparse: sparse: incorrect type in \
     argument 1 (different address spaces)
		   @@     expected void *dst
		   @@     got void [noderef] __iomem *dst @@
     arch/x86/include/asm/io.h:422:27: sparse:     expected void *dst
     arch/x86/include/asm/io.h:422:27: sparse:     got void [noderef] __iomem *dst

 [ bp: Massage commit message. ]

Fixes: 0888e10 ("x86/asm: Carve out a generic movdir64b() helper for general usage")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Ben Widawsky <ben.widawsky@intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Link: https://lkml.kernel.org/r/161003787823.4062451.6564503265464317197.stgit@djiang5-desk3.ch.intel.com

arch/x86/include/asm/special_insns.h

@@ -243,10 +243,10 @@ static inline void serialize(void)
 }
 
 /* The dst parameter must be 64-bytes aligned */
-static inline void movdir64b(void *dst, const void *src)
+static inline void movdir64b(void __iomem *dst, const void *src)
 {
 	const struct { char _[64]; } *__src = src;
-	struct { char _[64]; } *__dst = dst;
+	struct { char _[64]; } __iomem *__dst = dst;
 
 	/*
 	 * MOVDIR64B %(rdx), rax.