ext4: call sync_blockdev() before invalidate_bdev() in put_super()

Normally all of the buffers will have been forced out to disk before
we call invalidate_bdev(), but there will be some cases, where a file
system operation was aborted due to an ext4_error(), where there may
still be some dirty buffers in the buffer cache for the device.  So
try to force them out to memory before calling invalidate_bdev().

This fixes a warning triggered by generic/081:

WARNING: CPU: 1 PID: 3473 at /usr/projects/linux/ext4/fs/block_dev.c:56 __blkdev_put+0xb5/0x16f()
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org

fs/ext4/super.c

@@ -828,6 +828,7 @@ static void ext4_put_super(struct super_block *sb)
 		dump_orphan_list(sb, sbi);
 	J_ASSERT(list_empty(&sbi->s_orphan));
 
+	sync_blockdev(sb->s_bdev);
 	invalidate_bdev(sb->s_bdev);
 	if (sbi->journal_bdev && sbi->journal_bdev != sb->s_bdev) {
 		/*