xen/privcmd: Convert get_user_pages*() to pin_user_pages*()

In 2019, we introduced pin_user_pages*() and now we are converting
get_user_pages*() to the new API as appropriate. [1] & [2] could
be referred for more information. This is case 5 as per document [1].

[1] Documentation/core-api/pin_user_pages.rst

[2] "Explicit pinning of user-space pages":
        https://lwn.net/Articles/807108/Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Paul Durrant <xadimgnik@gmail.com>
Link: https://lore.kernel.org/r/1594525195-28345-4-git-send-email-jrdr.linux@gmail.comSigned-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/privcmd.c

@@ -594,7 +594,7 @@ static int lock_pages(
 		if (requested > nr_pages)
 			return -ENOSPC;
 
-		page_count = get_user_pages_fast(
+		page_count = pin_user_pages_fast(
 			(unsigned long) kbufs[i].uptr,
 			requested, FOLL_WRITE, pages);
 		if (page_count < 0)
@@ -610,13 +610,7 @@ static int lock_pages(
 
 static void unlock_pages(struct page *pages[], unsigned int nr_pages)
 {
-	unsigned int i;
-
-	for (i = 0; i < nr_pages; i++) {
-		if (!PageDirty(pages[i]))
-			set_page_dirty_lock(pages[i]);
-		put_page(pages[i]);
-	}
+	unpin_user_pages_dirty_lock(pages, nr_pages, true);
 }
 
 static long privcmd_ioctl_dm_op(struct file *file, void __user *udata)