Fix a potential integer overflow

Because nr_files and new_size are not same types.It can cause new_size
less than zero.Then realloc will be failed.

Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Remove repeated --readonly options in documentation

Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>