erofs: better naming for erofs inode related stuffs

updates inode naming
 - kill is_inode_layout_compression [1]
 - kill magic underscores [2] [3]
 - better naming for datamode & data_mapping_mode [3]
 - better naming erofs_inode_{compact, extended} [4]

[1] https://lore.kernel.org/r/20190829102426.GE20598@infradead.org/
[2] https://lore.kernel.org/r/20190829102426.GE20598@infradead.org/
[3] https://lore.kernel.org/r/20190902122627.GN15931@infradead.org/
[4] https://lore.kernel.org/r/20190902125438.GA17750@infradead.org/


Reported-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190904020912.63925-8-gaoxiang25@huawei.com


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