device-dax: fix private mapping restriction, permit read-only

Hugh notes in response to commit 4cb19355ea19 "device-dax: fail all
private mapping attempts":

"I think that is more restrictive than you intended: haven't tried, but I
believe it rejects a PROT_READ, MAP_SHARED, O_RDONLY fd mmap, leaving no
way to mmap /dev/dax without write permission to it."

Indeed it does restrict read-only mappings, switch to checking
VM_MAYSHARE, not VM_SHARED.

Cc: <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Pawel Lebioda <[e-mail]>
Fixes: 4cb19355ea19 ("device-dax: fail all private mapping attempts")
Reported-by: Hugh Dickins <[e-mail]>
Signed-off-by: Dan Williams <[e-mail]>

drivers/dax/dax.c

···
 		return -ENXIO;
 
 	/* prevent private mappings from being established */
-	if ((vma->vm_flags & VM_SHARED) != VM_SHARED) {
+	if ((vma->vm_flags & VM_MAYSHARE) != VM_MAYSHARE) {
 		dev_info(dev, "%s: %s: fail, attempted private mapping\n",
 				current->comm, func);
 		return -EINVAL;