f2fs: remove write attribute of main_blkaddr sysfs node

Fuzzing main_blkaddr sysfs node will corrupt this field's value,
causing kernel panic, remove its write attribute to avoid potential
security risk.

[Chao Yu: add description]

Signed-off-by: Dehe Gu <gudehe@huawei.com>
Signed-off-by: Daiyue Zhang <zhangdaiyue1@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>