
This is a simple routine to mimic the 4.x truncate system call.
The differences are:
	1:  You need file size + truncate to size free on the device the file resizes on.
	2:  you need to have the same euid and egid as the file to truncate (it could be smarter)
	3:  crashes would leave artifacts around (a tmp file)
	4:  worst case crash--- you could be left with only the tmpfile [with all the data you want in it].
		4.a:  This is a bit shaky, as it would have just finished writing into this file,  and it is probably not sync'd yet.

