MIPS: mm: Add compound tail page _mapcount when mapped

See commit b6999b19 which did the same modification for x86's mm/gup,

Quote from commit b6999b19:
    "If compound pages are used and the page is a
    tail page, gup_huge_pmd() increases _mapcount to record tail page are
    mapped while gup_huge_pud does not do that."

[ralf@linux-mips.org: fixed rejects caused by the original patch getting
linewrapped.]

Signed-off-by: Jovi Zhang <boojovi@gmail.com>
Cc: Youquan Song <youquan.song@intel.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: <stable@vger.kernel.org>
Patchwork: https://patchwork.linux-mips.org/patch/4291/


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