mm: add ksys_mmap_pgoff() helper; remove in-kernel calls to sys_mmap_pgoff()

Using this helper allows us to avoid the in-kernel calls to the
sys_mmap_pgoff() 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_mmap_pgoff().

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/m68k/kernel/sys_m68k.c

@@ -46,7 +46,7 @@ asmlinkage long sys_mmap2(unsigned long addr, unsigned long len,
 	 * so we need to shift the argument down by 1; m68k mmap64(3)
 	 * (in libc) expects the last argument of mmap2 in 4Kb units.
 	 */
-	return sys_mmap_pgoff(addr, len, prot, flags, fd, pgoff);
+	return ksys_mmap_pgoff(addr, len, prot, flags, fd, pgoff);
 }
 
 /* Convert virtual (user) address VADDR to physical address PADDR */