Commit 197997a broke handling of the offset
arguments to copy_file_range() when specified non-NULL.
The code fails to update the offsets and, as such, a loop like:
do {
len = copy_file_range(infd, &inpos, outfd, &outpos, SSIZE_MAX, 0);
} while (len > 0);
becomes an infinite loop, just doing the same copy over and
over again.
This patch fixes it.
The clause "(foffsets_locked || foffsets_set)" is not actually
needed for correctness, but I thought it made the code
a little more readable and might avoid some static
analyzer from throwing a "user before being set" for
the savinoff and savoutoff variables.