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/sparc/kernel/sys_sparc32.c

@@ -241,7 +241,7 @@ long compat_sys_fadvise64_64(int fd,
 
 long sys32_sync_file_range(unsigned int fd, unsigned long off_high, unsigned long off_low, unsigned long nb_high, unsigned long nb_low, unsigned int flags)
 {
-	return sys_sync_file_range(fd,
+	return ksys_sync_file_range(fd,
 				   (off_high << 32) | off_low,
 				   (nb_high << 32) | nb_low,
 				   flags);