[NTOSKRNL] Flush file to disk when deleting file mappings

[zefklop]

CORE-17627

Purpose

When closing a file, fastfat zeroes it out from ValidDataLength up to the end of the file. The ValidDataLength field is updated when the file content is actually written to disk. There is currently a race between the file-close path and the page out path, leading to potential file corruptions when the zeroing happens after the memory has been flushed to disk.

Fix this by actually flushing the file to disk when unmapping files, with file lock acquired. This way, the FS driver cannot zero out the tail of the file while we're actually flushing it to disk.