UPSTREAM: swiotlb: fail map correctly with failed io_tlb_default_mem

In the failure case of trying to use a buffer which we'd previously
failed to allocate, the "!mem" condition is no longer sufficient since
io_tlb_default_mem became static and assigned by default. Update the
condition to work as intended per the rest of that conversion.

Bug: 254441685
Fixes: 463e862a ("swiotlb: Convert io_default_tlb_mem to static allocation")
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
(cherry picked from commit c51ba246)
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: I2d422a479c4307ec01ab5d5850ce0154ab46fd73