arm64: mte: fix prctl(PR_GET_TAGGED_ADDR_CTRL) if TCF0=NONE

Previously we were always returning a tag inclusion mask of zero via
PR_GET_TAGGED_ADDR_CTRL if TCF0 was set to NONE. Fix it by making
the code for the NONE case match the others.
Signed-off-by: Peter Collingbourne <pcc@google.com>
Link: https://linux-review.googlesource.com/id/Iefbea66cf7d2b4c80b82f9639b9ea7f33f7fac53
Fixes: af5ce952 ("arm64: mte: Allow user control of the generated random tags via prctl()")
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20201203075110.2781021-1-pcc@google.comSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/mte.c

@@ -189,7 +189,8 @@ long get_mte_ctrl(struct task_struct *task)
 
 	switch (task->thread.sctlr_tcf0) {
 	case SCTLR_EL1_TCF0_NONE:
-		return PR_MTE_TCF_NONE;
+		ret |= PR_MTE_TCF_NONE;
+		break;
 	case SCTLR_EL1_TCF0_SYNC:
 		ret |= PR_MTE_TCF_SYNC;
 		break;