ext4: convert to mbcache2

The conversion is generally straightforward. The only 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 buffer lock.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
static inline int ext4_handle_dirty_xattr_block(handle_t *handle,
						struct inode *inode,
						struct buffer_head *bh)
{
	ext4_xattr_block_csum_set(inode, bh->b_blocknr, BHDR(bh));
	return ext4_handle_dirty_metadata(handle, inode, bh);
}

static inline int ext4_handle_dirty_xattr_block(handle_t *handle,
						struct inode *inode,
						struct buffer_head *bh)
{
	ext4_xattr_block_csum_set(inode, bh->b_blocknr, BHDR(bh));
	return ext4_handle_dirty_metadata(handle, inode, bh);
}
