ftruncate: pass a signed offset

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

ftruncate: pass a signed offset

The old ftruncate() syscall, using the 32-bit off_t misses a sign

extension when called in compat mode on 64-bit architectures. As a

result, passing a negative length accidentally succeeds in truncating

to file size between 2GiB and 4GiB.

Changing the type of the compat syscall to the signed compat_off_t

changes the behavior so it instead returns -EINVAL.

The native entry point, the truncate() syscall and the corresponding

loff_t based variants are all correct already and do not suffer

from this mistake.