swiotlb: Emit diagnostic in swiotlb_exit()

A recent debugging session would have been made a little bit easier if
we had noticed sooner that swiotlb_exit() was being called during boot.

Add a simple diagnostic message to swiotlb_exit() to complement the one
from swiotlb_print_info() during initialisation.

Cc: Claire Chang <tientzu@chromium.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/20210705190352.GA19461@willie-the-truck
Suggested-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Claire Chang <tientzu@chromium.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad@kernel.org>

kernel/dma/swiotlb.c

@@ -334,6 +334,7 @@ void __init swiotlb_exit(void)
 	if (!mem->nslabs)
 		return;
 
+	pr_info("tearing down default memory pool\n");
 	size = array_size(sizeof(*mem->slots), mem->nslabs);
 	if (mem->late_alloc)
 		free_pages((unsigned long)mem->slots, get_order(size));