BACKPORT: FROMLIST: arm64: mte: avoid TFSR related operations unless in async mode

There is no reason to touch TFSR nor issue a DSB unless our task is
in asynchronous mode. Since these operations (especially the DSB)
may be expensive on certain microarchitectures, only perform them
if necessary.

Furthermore, stop clearing TFSR on entry because it will be cleared
on exit and it is not necessary to have any particular value in TFSR
between entry and exit.

Signed-off-by: Peter Collingbourne <pcc@google.com>
Link: https://linux-review.googlesource.com/id/Ib353a63e3d0abc2b0b008e96aa2d9692cfc1b815

Change-Id: I9132dd39ce6f994ee993cae67b2476dfa73ef64a
Bug: 192536783
Link: https://lore.kernel.org/linux-arm-kernel/20210709023532.2133673-1-pcc@google.com/


Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
Signed-off-by: Peter Collingbourne <pcc@google.com>