fs: add ksys_sync_file_range helper(); remove in-kernel calls to syscall

Using this helper allows us to avoid the in-kernel calls to the
sys_sync_file_range() 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_sync_file_range().

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/mips/kernel/linux32.c

@@ -139,7 +139,7 @@ asmlinkage long sys32_sync_file_range(int fd, int __pad,
 	unsigned long a4, unsigned long a5,
 	int flags)
 {
-	return sys_sync_file_range(fd,
+	return ksys_sync_file_range(fd,
 			merge_64(a2, a3), merge_64(a4, a5),
 			flags);
 }