swiotlb: mark swiotlb_memblock_alloc() as __init

swiotlb_memblock_alloc() calls memblock_alloc(), which calls
(__init) memblock_alloc_try_nid(). However, swiotlb_membloc_alloc()
can be marked as __init since it is only called by swiotlb_init_remap(),
which is already marked as __init. This prevents a modpost build
warning/error:

WARNING: modpost: vmlinux.o: section mismatch in reference: swiotlb_memblock_alloc (section: .text) -> memblock_alloc_try_nid (section: .init.text)
WARNING: modpost: vmlinux.o: section mismatch in reference: swiotlb_memblock_alloc (section: .text) -> memblock_alloc_try_nid (section: .init.text)

This fixes the build warning/error seen on ARM64, PPC64, S390, i386,
and x86_64.

Fixes: 8d58aa48 ("swiotlb: reduce the swiotlb buffer size on allocation failure")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Alexey Kardashevskiy <aik@amd.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: iommu@lists.linux.dev
Cc: Mike Rapoport <rppt@kernel.org>
Cc: linux-mm@kvack.org
Signed-off-by: Christoph Hellwig <hch@lst.de>