ceph: invalidate dirty pages after forced umount

After forced umount, ceph_writepages_start() skips flushing dirty
pages. To make sure inode's reference count get dropped to zero,
we need to invalidate dirty pages.
Signed-off-by: Yan, Zheng <zyan@redhat.com>

fs/ceph/addr.c

@@ -719,6 +719,8 @@ static int ceph_writepages_start(struct address_space *mapping,
 
 	if (ACCESS_ONCE(fsc->mount_state) == CEPH_MOUNT_SHUTDOWN) {
 		pr_warn("writepage_start %p on forced umount\n", inode);
+		truncate_pagecache(inode, 0);
+		mapping_set_error(mapping, -EIO);
 		return -EIO; /* we're in a forced umount, don't write! */
 	}
 	if (fsc->mount_options->wsize && fsc->mount_options->wsize < wsize)