mm: khugepaged: skip huge page collapse for special files

In the Linux kernel, the following vulnerability has been resolved:

mm: khugepaged: skip huge page collapse for special files

The read-only THP for filesystems will collapse THP for files opened

readonly and mapped with VM_EXEC. The intended usecase is to avoid TLB

misses for large text segments. But it doesn't restrict the file types

so a THP could be collapsed for a non-regular file, for example, block

device, if it is opened readonly and mapped with EXEC permission. This

may cause bugs, like [1] and [2].

This is definitely not the intended usecase, so just collapse THP for

regular files in order to close the attack surface.

[[email protected]: fix vm_file check [3]]