erofs-utils: clean up erofs_write_file()

Introduce write_uncompressed_file_from_fd() to make
error handling path in erofs_write_file() clearer.

Link: https://lore.kernel.org/r/20191219181901.GA9405@hsiangkao-HP-ZHAN-66-Pro-G1
Signed-off-by: Li Guifu <blucerlee@gmail.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>