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>

include/linux/syscalls.h

@@ -1078,4 +1078,11 @@ static inline long ksys_open(const char __user *filename, int flags,
 	return do_sys_open(AT_FDCWD, filename, flags, mode);
 }
 
+extern long do_sys_truncate(const char __user *pathname, loff_t length);
+
+static inline long ksys_truncate(const char __user *pathname, loff_t length)
+{
+	return do_sys_truncate(pathname, length);
+}
+
 #endif