swiotlb: don't panic when the swiotlb buffer can't be allocated

For historical reasons the switlb code paniced when the metadata could
not be allocated, but just printed a warning when the actual main
swiotlb buffer could not be allocated.  Restore this somewhat unexpected
behavior as changing it caused a boot failure on the Microchip RISC-V
PolarFire SoC Icicle kit.

Fixes: 6424e31b

 ("swiotlb: remove swiotlb_init_with_tbl and swiotlb_init_late_with_tbl")
Reported-by: Conor Dooley <Conor.Dooley@microchip.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Tested-by: Conor Dooley <Conor.Dooley@microchip.com>