fsnotify: pass correct offset to fsnotify_mmap_perm()

fsnotify_mmap_perm() requires a byte offset for the file about to be
mmap'ed.  But it is called from vm_mmap_pgoff(), which has a page offset. 
Previously the conversion was done incorrectly so let's fix it, being
careful not to overflow on 32-bit platforms.

Discovered during code review.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 066e053 ("fsnotify: add pre-content hooks on mmap()")
Signed-off-by: Ryan Roberts <[email protected]>
Reviewed-by: Kiryl Shutsemau <[email protected]>
Cc: Amir Goldstein <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Liam Howlett <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: ryanhrob

mm/util.c

@@ -566,14 +566,15 @@ unsigned long vm_mmap_pgoff(struct file *file, unsigned long addr,
 	unsigned long len, unsigned long prot,
 	unsigned long flag, unsigned long pgoff)
 {
+	loff_t off = (loff_t)pgoff << PAGE_SHIFT;
 	unsigned long ret;
 	struct mm_struct *mm = current->mm;
 	unsigned long populate;
 	LIST_HEAD(uf);
 
 	ret = security_mmap_file(file, prot, flag);
 	if (!ret)
-		ret = fsnotify_mmap_perm(file, prot, pgoff >> PAGE_SHIFT, len);
+		ret = fsnotify_mmap_perm(file, prot, off, len);
 	if (!ret) {
 		if (mmap_write_lock_killable(mm))
 			return -EINTR;