mman: Fix errno handling in mmap and munmap

Nadav Har'El explains:

  Traditionally, functions which succeed do NOT set errno to zero, but
  rather leave it unchanged (errno(3) on Linux says, for example, that
  "errno is never set to zero by any system call or library function.").

Reviewed-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>