UPSTREAM: arm64: mm: Really fix sparse warning in untagged_addr()

(Upstream commit 9c1cac424c93d2b3122014e07a54b003ddedc168).

untagged_addr() can be called with a '__user' pointer parameter and must
therefore use '__force' casts both when passing this parameter through
to sign_extend64() as a 'u64', but also when casting the 's64' return
value back to the '__user' pointer type.

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Bug: 135692346
Change-Id: I8806aa6911cda3ab0489dff59b75ae214b02d48e