mm: add a call to flush_cache_vmap() in vmap_pfn()

flush_cache_vmap() must be called after new vmalloc mappings are installed
in the page table in order to allow architectures to make sure the new
mapping is visible.

It could lead to a panic since on some architectures (like powerpc),
the page table walker could see the wrong pte value and trigger a
spurious page fault that can not be resolved (see commit f1cb8f9b
("powerpc/64s/radix: avoid ptesync after set_pte and
ptep_set_access_flags")).

But actually the patch is aiming at riscv: the riscv specification
allows the caching of invalid entries in the TLB, and since we recently
removed the vmalloc page fault handling, we now need to emit a tlb
shootdown whenever a new vmalloc mapping is emitted
(https://lore.kernel.org/linux-riscv/20230725132246.817726-1-alexghiti@rivosinc.com/).
That's a temporary solution, there are ways to avoid that :)

Link: https://lkml.kernel.org/r/20230809164633.1556126-1-alexghiti@rivosinc.com
Fixes: 3e9a9e25 ("mm: add a vmap_pfn function")
Reported-by: Dylan Jhong <dylan@andestech.com>
Closes: https://lore.kernel.org/linux-riscv/ZMytNY2J8iyjbPPy@atctrx.andestech.com/Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Dylan Jhong <dylan@andestech.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/vmalloc.c

@@ -2979,6 +2979,10 @@ void *vmap_pfn(unsigned long *pfns, unsigned int count, pgprot_t prot)
 		free_vm_area(area);
 		return NULL;
 	}
+
+	flush_cache_vmap((unsigned long)area->addr,
+			 (unsigned long)area->addr + count * PAGE_SIZE);
+
 	return area->addr;
 }
 EXPORT_SYMBOL_GPL(vmap_pfn);