f2fs: optimize f2fs_write_cache_pages

The if statement "goto continue_unlock" is exactly the same when
each if condition is true that is depended on the value of both
"step" and "is_cold_data(page)" are 0 or 1. That means when the
value of "step" equals to "is_cold_data(page)", the if condition
is true and the if statement "goto continue_unlock" appears only
once, so it can be optimized to reduce the duplicated code.

Signed-off-by: Tiezhu Yang <kernelpatch@126.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>