hugetlb: check the return value of string conversion in sysctl handler

proc_doulongvec_minmax may fail if the given buffer doesn't represent a
valid number.  If we provide something invalid we will initialize the
resulting value (nr_overcommit_huge_pages in this case) to a random value
from the stack.

The issue was introduced by a3d0c6aa

 when the default handler has been
replaced by the helper function where we do not check the return value.

Reproducer:
echo "" > /proc/sys/vm/nr_overcommit_hugepages

[akpm@linux-foundation.org: correctly propagate proc_doulongvec_minmax return code]
Signed-off-by: Michal Hocko <mhocko@suse.cz>
Cc: CAI Qian <caiqian@redhat.com>
Cc: Nishanth Aravamudan <nacc@us.ibm.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>