ext2: convert to mbcache2

The conversion is generally straightforward. We convert filesystem from
a global cache to per-fs one. Similarly to ext4 the tricky part is that
xattr block corresponding to found mbcache entry can get freed before we
get buffer lock for that block. So we have to check whether the entry is
still valid after getting the buffer lock.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
static int ext2_unfreeze(struct super_block *sb)
{
	/* Just write sb to clear EXT2_VALID_FS flag */
	ext2_write_super(sb);

	return 0;
}

static int ext2_unfreeze(struct super_block *sb)
{
	/* Just write sb to clear EXT2_VALID_FS flag */
	ext2_write_super(sb);

	return 0;
}
