s390: claim efficient unaligned access

most unaligned accesses are reasonable efficient (no kernel emulation)
on s390, let's announce it

This also
- removes the ubsan false positives for unaligned accesses on s390 with
  default config
- uses simpler arithmetic in several functions in several other areas
  of the kernel like ethernet frame classification

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/Kconfig

@@ -137,6 +137,7 @@ config S390
 	select HAVE_DMA_API_DEBUG
 	select HAVE_DYNAMIC_FTRACE
 	select HAVE_DYNAMIC_FTRACE_WITH_REGS
+	select HAVE_EFFICIENT_UNALIGNED_ACCESS
 	select HAVE_EXIT_THREAD
 	select HAVE_FTRACE_MCOUNT_RECORD
 	select HAVE_FUNCTION_GRAPH_TRACER