syscalls: Fix offset type of ksys_ftruncate()

After the commit below, truncate() on x86 32bit uses ksys_ftruncate(). But
ksys_ftruncate() truncates the offset to unsigned long.

Switch the type of offset to loff_t which is what do_sys_ftruncate()
expects.

Fixes: 121b32a58a3a (x86/entry/32: Use IA32-specific wrappers for syscalls taking 64-bit arguments)
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Brian Gerst <brgerst@gmail.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20200610114851.28549-1-jslaby@suse.cz