LoongArch: refresh usage of sync

LoongArch inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I6BWFP



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sync is used for ll/sc pair for read barrier, instead only ll is
required for read barrier, sc does not need. This patch fixes
the issue.

Change-Id: Ib8a42f50d494e3e80cda0843d069668604b486e6
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
(cherry picked from commit d8911a28)