mm: add ksys_fadvise64_64() helper; remove in-kernel call to sys_fadvise64_64()

Using the ksys_fadvise64_64() helper allows us to avoid the in-kernel
calls to the sys_fadvise64_64() 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 ksys_fadvise64_64().

Some compat stubs called sys_fadvise64(), which then just passed through
the arguments to sys_fadvise64_64(). Get rid of this indirection, and call
ksys_fadvise64_64() directly.

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: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

arch/x86/ia32/sys_ia32.c

@@ -194,10 +194,10 @@ COMPAT_SYSCALL_DEFINE6(x86_fadvise64_64, int, fd, __u32, offset_low,
 		       __u32, offset_high, __u32, len_low, __u32, len_high,
 		       int, advice)
 {
-	return sys_fadvise64_64(fd,
-			       (((u64)offset_high)<<32) | offset_low,
-			       (((u64)len_high)<<32) | len_low,
-				advice);
+	return ksys_fadvise64_64(fd,
+				 (((u64)offset_high)<<32) | offset_low,
+				 (((u64)len_high)<<32) | len_low,
+				 advice);
 }
 
 COMPAT_SYSCALL_DEFINE4(x86_readahead, int, fd, unsigned int, off_lo,
@@ -218,8 +218,8 @@ COMPAT_SYSCALL_DEFINE6(x86_sync_file_range, int, fd, unsigned int, off_low,
 COMPAT_SYSCALL_DEFINE5(x86_fadvise64, int, fd, unsigned int, offset_lo,
 		       unsigned int, offset_hi, size_t, len, int, advice)
 {
-	return sys_fadvise64_64(fd, ((u64)offset_hi << 32) | offset_lo,
-				len, advice);
+	return ksys_fadvise64_64(fd, ((u64)offset_hi << 32) | offset_lo,
+				 len, advice);
 }
 
 COMPAT_SYSCALL_DEFINE6(x86_fallocate, int, fd, int, mode,