FROMLIST: arm64: mte: rename gcr_user_excl to mte_ctrl

We are going to use this field to store more data. To prepare for
that, rename it and change the users to rely on the bit position of
gcr_user_excl in mte_ctrl.

Link: https://linux-review.googlesource.com/id/Ie1fd18e480100655f5d22137f5b22f4f3a9f9e2e


Signed-off-by: Peter Collingbourne <pcc@google.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/linux-arm-kernel/20210702194110.2045282-2-pcc@google.com/
Bug: 189966263
Change-Id: Ie1fd18e480100655f5d22137f5b22f4f3a9f9e2e