hugetlbfs: add NULL check in hugetlb_zero_setup()

If hugetlbfs module_init() fails, hugetlbfs_vfsmount is not initialized and
shmget() with SHM_HUGETLB flag will cause NULL pointer dereference.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: William Irwin <wli@holomorphy.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/hugetlbfs/inode.c

@@ -747,6 +747,9 @@ struct file *hugetlb_zero_setup(size_t size)
 	char buf[16];
 	static atomic_t counter;
 
+	if (!hugetlbfs_vfsmount)
+		return ERR_PTR(-ENOENT);
+
 	if (!can_do_hugetlb_shm())
 		return ERR_PTR(-EPERM);