sched/preempt, sh: kmap_coherent relies on disabled preemption

kmap_coherent needs disabled preemption to not schedule in the critical
section, just like kmap_coherent on mips and kmap_atomic in general.

Fixes: 8222dbe2

 "sched/preempt, mm/fault: Decouple preemption from the page fault logic"
Reported-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Tested-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Rich Felker <dalias@libc.org>