arm64: unify idmap removal

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 <[email protected]>
Reviewed-by: Catalin Marinas <[email protected]>
Tested-by: Ard Biesheuvel <[email protected]>
Reviewed-by: Ard Biesheuvel <[email protected]>
Tested-by: Jeremy Linton <[email protected]>
Cc: Laura Abbott <[email protected]>
Cc: Lorenzo Pieralisi <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Catalin Marinas <[email protected]>