mm, gup: prevent pmd checking race in follow_pmd_mask()

mmap_sem will be read locked when calling follow_pmd_mask().  But this
cannot prevent PMD from being changed for all cases when PTL is
unlocked, for example, from pmd_trans_huge() to pmd_none() via
MADV_DONTNEED.  So it is possible for the pmd_present() check in
follow_pmd_mask() to encounter an invalid PMD.  This may cause an
incorrect VM_BUG_ON() or an infinite loop.  Fix this by reading the PMD
entry into a local variable with READ_ONCE() and checking the local
variable and pmd_none() in the retry loop.

As Kirill pointed out, with PTL unlocked, the *pmd may be changed under
us, so reading it directly again and again may incur weird bugs.  So
although using *pmd directly other than for pmd_present() checking may
be safe, it is still better to replace them to read *pmd once and check
the local variable multiple times.

When PTL unlocked, replace all *pmd with local variable was suggested by
Kirill.

Link: http://lkml.kernel.org/r/20180419083514.1365-1-ying.huang@intel.com
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Zi Yan <zi.yan@cs.rutgers.edu>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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