fs: add ksys_truncate() wrapper; remove in-kernel calls to sys_truncate()

Using the ksys_truncate() wrapper allows us to get rid of in-kernel
calls to the sys_truncate() syscall. The ksys_ prefix denotes that this
function is meant as a drop-in replacement for the syscall. In
particular, it uses the same calling convention as sys_truncate().

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

arch/x86/ia32/sys_ia32.c

@@ -55,7 +55,8 @@
 COMPAT_SYSCALL_DEFINE3(x86_truncate64, const char __user *, filename,
 		       unsigned long, offset_low, unsigned long, offset_high)
 {
-       return sys_truncate(filename, ((loff_t) offset_high << 32) | offset_low);
+	return ksys_truncate(filename,
+			    ((loff_t) offset_high << 32) | offset_low);
 }
 
 COMPAT_SYSCALL_DEFINE3(x86_ftruncate64, unsigned int, fd,