x86/asm: Add a missing __iomem annotation in enqcmds()

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.

Also, from the comment in movdir64b(), which also applies to enqcmds(),
@__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:

  drivers/dma/idxd/submit.c: note: in included file (through arch/x86/include/asm/processor.h, \
	arch/x86/include/asm/timex.h, include/linux/timex.h, include/linux/time32.h, \
	include/linux/time.h, include/linux/stat.h, ...):
  ./arch/x86/include/asm/special_insns.h:289:41: warning: incorrect type in initializer (different address spaces)
  ./arch/x86/include/asm/special_insns.h:289:41:    expected struct <noident> *__dst
  ./arch/x86/include/asm/special_insns.h:289:41:    got void [noderef] __iomem *dst

 [ bp: Massage commit message. ]

Fixes: 7f5933f8 ("x86/asm: Add an enqcmds() wrapper for the ENQCMDS instruction")
Reported-by: Nkernel test robot <lkp@intel.com>
Signed-off-by: NDave Jiang <dave.jiang@intel.com>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Reviewed-by: NBen Widawsky <ben.widawsky@intel.com>
Reviewed-by: NDan Williams <dan.j.williams@intel.com>
Link: https://lkml.kernel.org/r/161003789741.4062451.14362269365703761223.stgit@djiang5-desk3.ch.intel.com

arch/x86/include/asm/special_insns.h

@@ -286,7 +286,7 @@ static inline void movdir64b(void __iomem *dst, const void *src)
 static inline int enqcmds(void __iomem *dst, const void *src)
 {
 	const struct { char _[64]; } *__src = src;
-	struct { char _[64]; } *__dst = dst;
+	struct { char _[64]; } __iomem *__dst = dst;
 	int zf;
 
 	/*