swiotlb: fix default_swiotlb_limit() for non-growable default pool

Commit ad96ce3252db ("swiotlb: determine potential physical address
limit") changed default_swiotlb_limit() to return the maximum physical
address that could ever be used by dynamically allocated pools, so that
the value stays constant when CONFIG_SWIOTLB_DYNAMIC=y. However,
io_tlb_default_mem.phys_limit is returned even when the default pool
cannot grow, e.g. when swiotlb_init_remap() is called with a remap
callback.

This breaks Xen dom0 on x86: pci_xen_swiotlb_init() passes SWIOTLB_ANY
and xen_swiotlb_fixup(), so phys_limit is set to the top of dom0 memory,
although xen_swiotlb_fixup() makes the default pool below the 32-bit
boundary. xen_swiotlb_dma_supported() then rejects 32-bit DMA masks,
even though the bounce buffer is perfectly usable by such devices.

Return phys_limit only if the default pool can actually grow, otherwise
report the real end of the default pool, like in the
!CONFIG_SWIOTLB_DYNAMIC case.

Reported-by: Andreas Greve <andreas.greve@a-greve.de>
Reported-by: James Dingwall <james@dingwall.me.uk>
Fixes: ad96ce3252db ("swiotlb: determine potential physical address limit")
Closes: https://lore.kernel.org/xen-devel/f74668db-52fd-4575-8372-7bfdf10d62ac@a-greve.de/
Cc: stable@vger.kernel.org
Assisted-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Tested-by: James Dingwall <james@dingwall.me.uk>
Link: https://lore.kernel.org/all/20261005070518.1590097-1-m.szyprowski@samsung.com/
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>