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 a137e1cc ("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

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>