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/powerpc/kernel/syscalls.c

@@ -57,7 +57,7 @@ static inline long do_mmap2(unsigned long addr, size_t len,
 		off >>= shift;
 	}
 
-	ret = sys_mmap_pgoff(addr, len, prot, flags, fd, off);
+	ret = ksys_mmap_pgoff(addr, len, prot, flags, fd, off);
 out:
 	return ret;
 }