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>