arm64: unify idmap removal

We currently open-code the removal of the idmap and restoration of the
current task's MMU state in a few places.

Before introducing yet more copies of this sequence, unify these to call
a new helper, cpu_uninstall_idmap.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Jeremy Linton <jeremy.linton@arm.com>
Cc: Laura Abbott <labbott@fedoraproject.org>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/mm/mmu.c

@@ -468,9 +468,7 @@ void __init paging_init(void)
 	 * TTBR0 is only used for the identity mapping at this stage. Make it
 	 * point to zero page to avoid speculatively fetching new entries.
 	 */
-	cpu_set_reserved_ttbr0();
-	local_flush_tlb_all();
-	cpu_set_default_tcr_t0sz();
+	cpu_uninstall_idmap();
 }
 
 /*