mm/vmalloc: fix the unchecked dereference warning in vread_iter()

LKP reported smatch warning as below:

===================
smatch warnings:
mm/vmalloc.c:3689 vread_iter() error: we previously assumed 'vm' could be null (see line 3667)
......
06c8994  @3667 size = vm ? get_vm_area_size(vm) : va_size(va);
......
06c8994  @3689 else if (!(vm->flags & VM_IOREMAP))
                                 ^^^^^^^^^
Unchecked dereference
=====================

This is not a runtime bug because the possible null 'vm' in the
pointed place could only happen when flags == VMAP_BLOCK.  However, the
case 'flags == VMAP_BLOCK' should never happen and has been detected
with WARN_ON.  Please check vm_map_ram() implementation and the earlier
checking in vread_iter() at below:

                ~~~~~~~~~~~~~~~~~~~~~~~~~~
                /*
                 * VMAP_BLOCK indicates a sub-type of vm_map_ram area, need
                 * be set together with VMAP_RAM.
                 */
                WARN_ON(flags == VMAP_BLOCK);

                if (!vm && !flags)
                        continue;
                ~~~~~~~~~~~~~~~~~~~~~~~~~~

So add checking on whether 'vm' could be null when dereferencing it in
vread_iter(). This mutes smatch complaint.

Link: https://lkml.kernel.org/r/ZTCURc8ZQE+KrTvS@MiWiFi-R3L-srv
Link: https://lkml.kernel.org/r/ZS/2k6DIMd0tZRgK@MiWiFi-R3L-srv
Signed-off-by: Baoquan He <[email protected]>
Reported-by: kernel test robot <[email protected]>
Reported-by: Dan Carpenter <[email protected]>
Closes: https://lore.kernel.org/r/[email protected]/
Cc: Lorenzo Stoakes <[email protected]>
Cc: Philip Li <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Baoquan He

mm/vmalloc.c

@@ -3809,7 +3809,7 @@ long vread_iter(struct iov_iter *iter, const char *addr, size_t count)
 
 		if (flags & VMAP_RAM)
 			copied = vmap_ram_vread_iter(iter, addr, n, flags);
-		else if (!(vm->flags & VM_IOREMAP))
+		else if (!(vm && (vm->flags & VM_IOREMAP)))
 			copied = aligned_vread_iter(iter, addr, n);
 		else /* IOREMAP area is treated as memory hole */
 			copied = zero_iter(iter, n);