LoongArch: Make compute_return_era() return void

mainline inclusion
from mainline-v5.19-rc4
commit ea18d434
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5OHOB


CVE: NA

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compute_return_era() always returns 0, make it return void,
and then no need to check its return value for its callers.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>