MIPS: Add fast get_user_pages

Gup is used in a few cases, say futex.

This work is derived from the x86 version, and operations of pte and pmd are
adapted to the defines of MIPS in straight forward manner.

[ralf@linux-mips.org: Fixed up reject in arch/mips/mm/Makefile due to
whitespace formatting differences.  Fixed build error in gup.c due to
conflicting changes elsewhere in the kernel.]

Signed-off-by: Hillf Danton <dhillf@gmail.com>
Cc: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2859/


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