swiotlb: Free tbl memory in swiotlb_exit()

Although swiotlb_exit() frees the 'slots' metadata array referenced by
'io_tlb_default_mem', it leaves the underlying buffer pages allocated
despite no longer being usable.

Extend swiotlb_exit() to free the buffer pages as well as the slots
array.

Cc: Claire Chang <tientzu@chromium.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: 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>