arm64: mm: Rename asid2idx() to ctxid2asid()

The commit 0c8ea531 ("arm64: mm: Allocate ASIDs in pairs") introduce
the asid2idx and idx2asid macro, but these macros are not really useful
after the commit f88f42f8

 ("arm64: context: Free up kernel ASIDs if
KPTI is not in use").

The code "(asid & ~ASID_MASK)" can be instead by a macro, which is the
same code with asid2idx(). So rename it to ctxid2asid() for a better
understanding.

Also we add asid2ctxid() macro, the contextid can be generated based on
the asid and generation through this macro.

Signed-off-by: Yunfeng Ye <yeyunfeng@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Link: https://lore.kernel.org/r/c31516eb-6d15-94e0-421c-305fc010ea79@huawei.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>