um: Don't rename vmap to kernel_vmap

um: Don't rename vmap to kernel_vmap

In order to work around the existence of a vmap symbol in libpcap, the
UML makefile unconditionally redefines vmap to kernel_vmap. However,
this not only affects the actual vmap symbol, but also anything else
named vmap, including a number of struct members in DRM.

This would not be too much of a problem, since all uses are also
updated, except we now have Rust DRM bindings, which expect the
corresponding Rust structs to have 'vmap' names. Since the redefinition
applies in bindgen, but not to Rust code, we end up with errors such as:

error[E0560]: struct `drm_gem_object_funcs` has no fields named `vmap`
  --> rust/kernel/drm/gem/mod.rs:210:9

Since libpcap support was removed in commit 12b8e7e69aa7 ("um: Remove
obsolete pcap driver"), remove the, now unnecessary, define as well.

We also take this opportunity to update the comment.

Signed-off-by: David Gow <davidgow@google.com>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://patch.msgid.link/20251122083213.3996586-1-davidgow@google.com
Fixes: 12b8e7e69aa7 ("um: Remove obsolete pcap driver")
[adjust commmit message a bit]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>