powerpc/book3s64/pkeys: Optimize KUAP and KUEP feature disabled case

powerpc/book3s64/pkeys: Optimize KUAP and KUEP feature disabled case

If FTR_BOOK3S_KUAP is disabled, kernel will continue to run with the same AMR
value with which it was entered. Hence there is a high chance that
we can return without restoring the AMR value. This also helps the case
when applications are not using the pkey feature. In this case, different
applications will have the same AMR values and hence we can avoid restoring
AMR in this case too.

Also avoid isync() if not really needed.

Do the same for IAMR.

null-syscall benchmark results:

With smap/smep disabled:
Without patch:
	957.95 ns    2778.17 cycles
With patch:
	858.38 ns    2489.30 cycles

With smap/smep enabled:
Without patch:
	1017.26 ns    2950.36 cycles
With patch:
	1021.51 ns    2962.44 cycles

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201127044424.40686-23-aneesh.kumar@linux.ibm.com