f2fs-tools: fix to set c.auto_fix only for fsck

add_default_options() will be reused by fsck family tools, including
dump, dfrag, resize, sload, label, inject, add a comment for this.

And also fix to set c.auto_fix only for fsck in add_default_options().

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>