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)
......
06c89946  @3667 size = vm ? get_vm_area_size(vm) : va_size(va);
......
06c89946

  @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 <bhe@redhat.com>
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/202310171600.WCrsOwFj-lkp@intel.com/
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Philip Li <philip.li@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>