docs: filesystem: Update smaps vm flag list to latest

We've missed a few documentation when adding new VM_* flags.  Add the missing
pieces so they'll be in sync now.

Signed-off-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/20210302000646.432358-1-peterx@redhat.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>