arm64: uaccess: remove redundant PAN toggling

mainline inclusion
from mainline-v5.11-rc1
commit 7cf283c7
category: performance
bugzilla: https://e.gitee.com/open_euler/issues/list?issue=I4SCW7


CVE: NA

-------------------------------------------------------------------------

Some code (e.g. futex) needs to make privileged accesses to userspace
memory, and uses uaccess_{enable,disable}_privileged() in order to
permit this. All other uaccess primitives use LDTR/STTR, and never need
to toggle PAN.

Remove the redundant PAN toggling.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20201202131558.39270-12-mark.rutland@arm.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>