Linux: sync mapped data on umount

Since we have to disable zpl_prune_sb when umounting,
because the shrinker gets freed before zpl_kill_sb is
ever called, the umount might be unable to sync open files.

Let's do it in zfs_preumount.

Signed-off-by: Pavel Snajdr <snajpa@snajpa.net>

module/os/linux/zfs/zfs_vfsops.c

@@ -1546,9 +1546,25 @@ void
 zfs_preumount(struct super_block *sb)
 {
 	zfsvfs_t *zfsvfs = sb->s_fs_info;
+	znode_t *zp;
 
 	/* zfsvfs is NULL when zfs_domount fails during mount */
 	if (zfsvfs) {
+		/*
+		 * Since we have to disable zpl_prune_sb when umounting,
+		 * because the shrinker gets freed before zpl_kill_sb is
+		 * ever called, the umount might be unable to sync open files.
+		 *
+		 * Let's do it here.
+		 */
+		mutex_enter(&zfsvfs->z_znodes_lock);
+		for (zp = list_head(&zfsvfs->z_all_znodes); zp;
+		    zp = list_next(&zfsvfs->z_all_znodes, zp)) {
+			if (zp->z_sa_hdl)
+				filemap_write_and_wait(ZTOI(zp)->i_mapping);
+		}
+		mutex_exit(&zfsvfs->z_znodes_lock);
+
 		zfs_unlinked_drain_stop_wait(zfsvfs);
 		zfsctl_destroy(sb->s_fs_info);
 		/*