dma-debug: fix debugfs initialization order

Due to link order, dma_debug_init is called before debugfs has a chance
to initialize (via debugfs_init which also happens in the core initcall
stage), so the directories for dma-debug are never created.

Decouple dma_debug_fs_init from dma_debug_init and defer its init until
core_initcall_sync (after debugfs has been initialized) while letting
dma-debug initialization occur as soon as possible to catch any early
mappings, as suggested in [1].

[1] https://lore.kernel.org/linux-iommu/YIgGa6yF%2Fadg8OSN@kroah.com/



Fixes: 15b28bbc ("dma-debug: move initialization to common code")
Signed-off-by: Anthony Iliopoulos <ailiop@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>