arm64/mm: Add ptep_get_and_clear_full() to optimize process teardown

ptep_get_and_clear_full() adds a 'full' parameter which is not present
for the fallback ptep_get_and_clear() function. 'full' is set to 1 when
a full address space teardown is in progress. We use this information to
optimize arm64_sys_exit_group() by avoiding unfolding (and therefore
tlbi) contiguous ranges. Instead we just clear the PTE but allow all the
contiguous neighbours to keep their contig bit set, because we know we
are about to clear the rest too.

Before this optimization, the cost of arm64_sys_exit_group() exploded to
32x what it was before PTE_CONT support was wired up, when compiling the
kernel. With this optimization in place, we are back down to the
original cost.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>