[ENHANCEMENT] introduce secure way of handling files

[ktanska]

Files opened by mdadm should be securely open. One (bad) example is check_one_sharer() function. The problem is that fopen does not put the lock on the file/path.
After opening the file, one can do following things:

• alter or delete the file,

• delete /run/mdadm and make it a soft link to another directory containing file with same name

Goal here is to analyze it and fix this issue globally so 3rd party access to the file is locked. Probably, this can be achieved with fstat.

In current implementation, opened file value is stored locally, messing with the file does not change its stored value.
Proposed scenario:

1. Lock file (and path)
2. Run checks
3. Open file

[ktanska]

Process files can be used here to see if file is not removed or changed to another:

[ncroxon]

Not all files need to be locked. Some inside /sys/block/mdx/md/ can be modified.

otherwise, immutable will work:
if (ioctl(fd, FS_IOC_GETFLAGS, &flags) == -1) {
perror("ioctl get flags");
return 1;
}

flags |= FS_IMMUTABLE_FL;
if (ioctl(fd, FS_IOC_SETFLAGS, &flags) == -1) {
perror("ioctl set flags");
return 1;
}