arm64: hugetlb: Avoid unnecessary clearing in huge_ptep_set_access_flags

For contiguous hugetlb, huge_ptep_set_access_flags performs a
get_clear_flush (which then flushes the TLBs) even when no change of ptes
is necessary.

Unfortunately, this behaviour can lead to back-to-back page faults being
generated when running with multiple threads that access the same
contiguous huge page.

Thread 1                     |  Thread 2
-----------------------------+------------------------------
hugetlb_fault                |
huge_ptep_set_access_flags   |
  -> invalidate pte range    | hugetlb_fault
continue processing          | wait for hugetlb_fault_mutex
release mutex and return     | huge_ptep_set_access_flags
                             |   -> invalidate pte range
hugetlb_fault
...

This patch changes huge_ptep_set_access_flags s.t. we first read the
contiguous range of ptes (whilst preserving dirty information); the pte
range is only then invalidated where necessary and this prevents further
spurious page faults.

Fixes: d8bdcff2

 ("arm64: hugetlb: Add break-before-make logic for contiguous entries")
Reported-by: Lei Zhang <zhang.lei@jp.fujitsu.com>
Signed-off-by: Steve Capper <steve.capper@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>