f2fs-tools: deal with permission denial on non-root user

This fixes some android build failures due to the missing permission when
checking the loop device. Until we get a better solution, let's ignore
the error with warnings.

Reviewed-by: Huang Jianan <huangjianan@xiaomi.com>
Reviewed-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>