f2fs: fix 3 coding style errors in f2fs.h

Two coding style errors below have been resolved:
"Macros with complex values should be enclosed in parentheses"

And a coding style error below has been resolved:
"space prohibited before that ',' (ctx:WxW)"

Signed-off-by: DongOh Shin <doscode.kr@gmail.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>