f2fs: fix to convert log type to segment data type correctly

This patch introduces a new helper log_type_to_seg_type() to convert
log type to segment data type, and uses it to clean up opened codes
in build_curseg(), and also it fixes to convert log type before use
in do_write_page().

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>