mm/hugetlb.c: add missing spin_lock() to hugetlb_cow()

mm/hugetlb.c: add missing spin_lock() to hugetlb_cow()

commit 44e2aa937e698ea95dd86b2a4fabd734ef2c76db upstream.

Add missing spin_lock() of the page_table_lock before an error return in
hugetlb_cow(). Callers of hugtelb_cow() expect it to be held upon return.

Signed-off-by: Dean Nelson <dnelson@redhat.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>