arm64/mm: Implement clear_ptes() to optimize exit, munmap, dontneed

With the core-mm changes in place to batch-clear ptes during
zap_pte_range(), we can take advantage of this in arm64 to greatly
reduce the number of tlbis we have to issue, and recover the lost
performance in exit, munmap and madvise(DONTNEED) incured when adding
support for transparent contiguous ptes.

If we are clearing a whole contpte range, we can elide first unfolding
that range and save the tlbis. We just clear the whole range.

The following microbenchmark results demonstate the effect of this
change on madvise(DONTNEED) performance for large pte-mapped folios.
madvise(dontneed) is called for each page of a 1G populated mapping and
the total time is measured. 100 iterations per run, 8 runs performed on
both Apple M2 (VM) and Ampere Altra (bare metal). Tests performed for
case where 1G memory is comprised of pte-mapped order-9 folios. Negative
is faster, positive is slower, compared to baseline upon which the
series is based:

| dontneed      |    Apple M2 VM    |    Ampere Altra   |
| order-9       |-------------------|-------------------|
| (pte-map)     |    mean |   stdev |    mean |   stdev |
|---------------|---------|---------|---------|---------|
| baseline      |    0.0% |    7.9% |    0.0% |    0.0% |
| before-change |   -1.3% |    7.0% |   13.0% |    0.0% |
| after-change  |   -9.9% |    0.9% |   14.1% |    0.0% |

The memory is initially all contpte-mapped and has to be unfolded (which
requires tlbi for the whole block) when the first page is touched (since
the test is madvise-ing 1 page at a time). Ampere Altra has high cost
for tlbi; this is why cost increases there.

The following microbenchmark results demonstate the recovery (and
overall improvement) of munmap performance for large pte-mapped folios.
munmap is called for a 1G populated mapping and the function runtime is
measured. 100 iterations per run, 8 runs performed on both Apple M2 (VM)
and Ampere Altra (bare metal). Tests performed for case where 1G memory
is comprised of pte-mapped order-9 folios. Negative is faster, positive
is slower, compared to baseline upon which the series is based:

| munmap        |    Apple M2 VM    |    Ampere Altra   |
| order-9       |-------------------|-------------------|
| (pte-map)     |    mean |   stdev |    mean |   stdev |
|---------------|---------|---------|---------|---------|
| baseline      |    0.0% |    6.4% |    0.0% |    0.1% |
| before-change |   43.3% |    1.9% |  375.2% |    0.0% |
| after-change  |   -6.0% |    1.4% |   -0.6% |    0.2% |

Tested-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>