dax: fix vma_is_fsdax() helper

Gerd reports that ->i_mode may contain other bits besides S_IFCHR. Use
S_ISCHR() instead. Otherwise, get_user_pages_longterm() may fail on
device-dax instances when those are meant to be explicitly allowed.

Fixes: 2bb6d28 ("mm: introduce get_user_pages_longterm")
Cc: <[email protected]>
Reported-by: Gerd Rausch <[email protected]>
Acked-by: Jane Chu <[email protected]>
Reported-by: Haozhong Zhang <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Signed-off-by: Dan Williams <[email protected]>

Author: djbw

include/linux/fs.h

@@ -3198,7 +3198,7 @@ static inline bool vma_is_fsdax(struct vm_area_struct *vma)
 	if (!vma_is_dax(vma))
 		return false;
 	inode = file_inode(vma->vm_file);
-	if (inode->i_mode == S_IFCHR)
+	if (S_ISCHR(inode->i_mode))
 		return false; /* device-dax */
 	return true;
 }