kselftest/arm64: Verify that TCO is enabled in load_unaligned_zeropad()

load_unaligned_zeropad() and __get/put_kernel_nofault() functions can
read passed some buffer limits which may include some MTE granule with a
different tag.

When MTE async mode is enable, the load operation crosses the boundaries
and the next granule has a different tag the PE sets the TFSR_EL1.TF1
bit as if an asynchronous tag fault is happened:

 ==================================================================
 BUG: KASAN: invalid-access
 Asynchronous mode enabled: no access details available

 CPU: 0 PID: 1 Comm: init Not tainted 5.12.0-rc1-ge1045c86620d-dirty #8
 Hardware name: FVP Base RevC (DT)
 Call trace:
   dump_backtrace+0x0/0x1c0
   show_stack+0x18/0x24
   dump_stack+0xcc/0x14c
   kasan_report_async+0x54/0x70
   mte_check_tfsr_el1+0x48/0x4c
   exit_to_user_mode+0x18/0x38
   finish_ret_to_user+0x4/0x15c
 ==================================================================

Verify that Tag Check Override (TCO) is enabled in these functions before
the load and disable it afterwards to prevent this to happen.

Note: The issue has been observed only with an MTE enabled userspace.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Reported-by: Branislav Rankov <Branislav.Rankov@arm.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>