f2fs: avoid swapon failure by giving a warning first

The final solution can be migrating blocks to form a section-aligned file
internally. Meanwhile, let's ask users to do that when preparing the swap
file initially like:
1) create()
2) ioctl(F2FS_IOC_SET_PIN_FILE)
3) fallocate()

Reported-by: kernel test robot <oliver.sang@intel.com>
Fixes: 36e4d958 ("f2fs: check if swapfile is section-alligned")
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>