f2fs: Simplify bool conversion

Fix the following coccicheck warning:
./fs/f2fs/sysfs.c:491:41-46: WARNING: conversion to bool not needed here

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>