hugetlbfs: use helper macro default_hstate in init_hugetlbfs_fs

Since commit e5ff2159

 ("hugetlb: multiple hstates for multiple page
sizes"), we can use macro default_hstate to get the struct hstate which we
use by default.  But init_hugetlbfs_fs() forgot to use it.

Link: https://lkml.kernel.org/r/20210116091827.20982-1-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>