MIPS: syscall: Return directly in mips_mmap()

* Return an error code without storing it in an intermediate variable.

* Delete the local variable "result" which became unnecessary with
  this refactoring.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: linux-mips@linux-mips.org
Cc: LKML <linux-kernel@vger.kernel.org>
Cc: kernel-janitors@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/15073/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>