iommu: Convert unreachable() to BUG()

iommu: Convert unreachable() to BUG()

Bare unreachable() should be avoided as it generates undefined behavior,
e.g. falling through to the next function.  Use BUG() instead so the
error is defined.

Fixes the following warnings:

  drivers/iommu/dma-iommu.o: warning: objtool: iommu_dma_sw_msi+0x92: can't find jump dest instruction at .text+0x54d5
  vmlinux.o: warning: objtool: iommu_dma_get_msi_page() falls through to next function __iommu_dma_unmap()

Link: https://patch.msgid.link/r/0c801ae017ec078cacd39f8f0898fc7780535f85.1743053325.git.jpoimboe@kernel.org
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Closes: https://lore.kernel.org/314f8809-cd59-479b-97d7-49356bf1c8d1@infradead.org
Reported-by: Paul E. McKenney <paulmck@kernel.org>
Closes: https://lore.kernel.org/5dd1f35e-8ece-43b7-ad6d-86d02d2718f6@paulmck-laptop
Fixes: 6aa63a4ec947 ("iommu: Sort out domain user data")
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>