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>

arch/sparc/kernel/sys_sparc32.c

@@ -65,7 +65,7 @@ asmlinkage long sys32_ftruncate64(unsigned int fd, unsigned long high, unsigned
 	if ((int)high < 0)
 		return -EINVAL;
 	else
-		return sys_ftruncate(fd, (high << 32) | low);
+		return ksys_ftruncate(fd, (high << 32) | low);
 }
 
 static int cp_compat_stat64(struct kstat *stat,