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/powerpc/kernel/sys_ppc32.c

@@ -124,5 +124,5 @@ asmlinkage long compat_sys_sync_file_range2(int fd, unsigned int flags,
 	loff_t offset = ((loff_t)offset_hi << 32) | offset_lo;
 	loff_t nbytes = ((loff_t)nbytes_hi << 32) | nbytes_lo;
 
-	return sys_sync_file_range(fd, offset, nbytes, flags);
+	return ksys_sync_file_range(fd, offset, nbytes, flags);
 }