mm/vmalloc.c: allow vread() to read out vm_map_ram areas

mm/vmalloc.c: allow vread() to read out vm_map_ram areas

Currently, vread can read out vmalloc areas which is associated with a
vm_struct.  While this doesn't work for areas created by vm_map_ram()
interface because it doesn't have an associated vm_struct.  Then in
vread(), these areas are all skipped.

Here, add a new function vmap_ram_vread() to read out vm_map_ram areas.
The area created with vmap_ram_vread() interface directly can be handled
like the other normal vmap areas with aligned_vread().  While areas which
will be further subdivided and managed with vmap_block need carefully read
out page-aligned small regions and zero fill holes.

Link: https://lkml.kernel.org/r/[email protected]
Reported-by: Stephen Brennan <[email protected]>
Signed-off-by: Baoquan He <[email protected]>
Reviewed-by: Lorenzo Stoakes <[email protected]>
Tested-by: Stephen Brennan <[email protected]>
Cc: Dan Carpenter <[email protected]>
Cc: Uladzislau Rezki (Sony) <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>