hugetlbfs: flush before unlock on move_hugetlb_page_tables()

We must flush the TLB before releasing i_mmap_rwsem to avoid the
potential reuse of an unshared PMDs page.  This is not true in the case
of move_hugetlb_page_tables().  The last reference on the page table can
therefore be dropped before the TLB flush took place.

Prevent it by reordering the operations and flushing the TLB before
releasing i_mmap_rwsem.

Fixes: 550a7d60 ("mm, hugepages: add mremap() support for hugepage backed vma")
Signed-off-by: Nadav Amit <namit@vmware.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mina Almasry <almasrymina@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>