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/x86/ia32/sys_ia32.c

@@ -209,9 +209,9 @@ COMPAT_SYSCALL_DEFINE6(x86_sync_file_range, int, fd, unsigned int, off_low,
 		       unsigned int, off_hi, unsigned int, n_low,
 		       unsigned int, n_hi, int, flags)
 {
-	return sys_sync_file_range(fd,
-				   ((u64)off_hi << 32) | off_low,
-				   ((u64)n_hi << 32) | n_low, flags);
+	return ksys_sync_file_range(fd,
+				    ((u64)off_hi << 32) | off_low,
+				    ((u64)n_hi << 32) | n_low, flags);
 }
 
 COMPAT_SYSCALL_DEFINE5(x86_fadvise64, int, fd, unsigned int, offset_lo,