iommu: Convert unreachable() to BUG()

JIRA: https://issues.redhat.com/browse/RHEL-114131
Upstream-Status: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git

commit 3a2ffd3
Author: Josh Poimboeuf <jpoimboe@kernel.org>
Date: Wed Mar 26 22:28:46 2025 -0700

    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: 6aa63a4 ("iommu: Sort out domain user data")
    Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
    Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

(cherry picked from commit 3a2ffd3)
Signed-off-by: Jerry Snitselaar <jsnitsel@redhat.com>

Author: snits

drivers/iommu/dma-iommu.c

@@ -2123,7 +2123,7 @@ static size_t cookie_msi_granule(const struct iommu_domain *domain)
 	case IOMMU_COOKIE_DMA_MSI:
 		return PAGE_SIZE;
 	default:
-		unreachable();
+		BUG();
 	};
 }
 
@@ -2135,7 +2135,7 @@ static struct list_head *cookie_msi_pages(const struct iommu_domain *domain)
 	case IOMMU_COOKIE_DMA_MSI:
 		return &domain->msi_cookie->msi_page_list;
 	default:
-		unreachable();
+		BUG();
 	};
 }