hugetlbfs: use truncate mutex to prevent pmd sharing race

hugetlbfs: use truncate mutex to prevent pmd sharing race

The synchronization mechanism for hugetlbfs pagefaults/truncation and
pmd sharing ideally needs to be modified to use i_mmap_rwsem.  See:
http://lkml.kernel.org/r/20181024045053.1467-1-mike.kravetz@oracle.com

In UEK, we have introduced a hugetlbfs truncate mutex in an inode
extension.  By taking this mutex earlier in hugetlb_fault (before calling
huge_pte_alloc), we eliminate the most common cause of problems where
ptep can be altered by a call to huge_pmd_unshare.

Orabug: 28896255

Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Larry Bassel <larry.bassel@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>