arm64/mm: Elide TLB flush in certain pte protection transitions

arm64/mm: Elide TLB flush in certain pte protection transitions

Currently arm64 does an unconditional TLB flush in mprotect(). This is not
required for some cases, for example, when changing from PROT_NONE to
PROT_READ | PROT_WRITE (a real usecase - glibc malloc does this to emulate
growing into the non-main heaps), and unsetting uffd-wp in a range.

Therefore, implement pte_needs_flush() for arm64, which is already
implemented by some other arches as well.

Running a userspace program changing permissions back and forth between
PROT_NONE and PROT_READ | PROT_WRITE, and measuring the average time taken
for the none->rw transition, I get a reduction from 3.2 microseconds to
2.85 microseconds, giving a 12.3% improvement.

Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>