fs: add ksys_ftruncate() wrapper; remove in-kernel calls to sys_ftruncate()

Using the ksys_ftruncate() wrapper allows us to get rid of in-kernel
calls to the sys_ftruncate() 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_ftruncate().

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>
diff --git a/init/initramfs.c b/init/initramfs.c
index 35173be..0d3b001 100644
--- a/init/initramfs.c
+++ b/init/initramfs.c
@@ -346,7 +346,7 @@ static int __init do_name(void)
 				ksys_fchown(wfd, uid, gid);
 				ksys_fchmod(wfd, mode);
 				if (body_len)
-					sys_ftruncate(wfd, body_len);
+					ksys_ftruncate(wfd, body_len);
 				vcollected = kstrdup(collected, GFP_KERNEL);
 				state = CopyFile;
 			}