mm/hugetlb: fix total hugetlbfs pages count when using memory overcommit accouting

mm/hugetlb: fix total hugetlbfs pages count when using memory overcommit accouting

hugetlb_total_pages is used for overcommit calculations but the current
implementation considers only the default hugetlb page size (which is
either the first defined hugepage size or the one specified by
default_hugepagesz kernel boot parameter).

If the system is configured for more than one hugepage size, which is
possible since commit a137e1cc6d6e ("hugetlbfs: per mount huge page
sizes") then the overcommit estimation done by __vm_enough_memory()
(resp.  shown by meminfo_proc_show) is not precise - there is an
impression of more available/allowed memory.  This can lead to an
unexpected ENOMEM/EFAULT resp.  SIGSEGV when memory is accounted.

Testcase:
  boot: hugepagesz=1G hugepages=1
  the default overcommit ratio is 50
  before patch:

    egrep 'CommitLimit' /proc/meminfo
    CommitLimit:     55434168 kB

  after patch:

    egrep 'CommitLimit' /proc/meminfo
    CommitLimit:     54909880 kB

[akpm@linux-foundation.org: coding-style tweak]
Signed-off-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: Hillf Danton <dhillf@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: <stable@vger.kernel.org>		[3.0+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>