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

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>