erofs: update erofs_inode_is_data_compressed helper

As Christoph said, "This looks like a really obsfucated
way to write:
	return datamode == EROFS_INODE_FLAT_COMPRESSION ||
		datamode == EROFS_INODE_FLAT_COMPRESSION_LEGACY; "

Although I had my own consideration, it's the right way for now.

[1] https://lore.kernel.org/r/20190829095954.GB20598@infradead.org/


Reported-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190904020912.63925-6-gaoxiang25@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>