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>

arch/sh/mm/kmap.c

@@ -36,6 +36,7 @@ void *kmap_coherent(struct page *page, unsigned long addr)
 
 	BUG_ON(!test_bit(PG_dcache_clean, &page->flags));
 
+	preempt_disable();
 	pagefault_disable();
 
 	idx = FIX_CMAP_END -
@@ -64,4 +65,5 @@ void kunmap_coherent(void *kvaddr)
 	}
 
 	pagefault_enable();
+	preempt_enable();
 }