udev: restore compat symlink for nvme devices

udev: restore compat symlink for nvme devices

In 5118e8e71dda211d20e34ec8d3012186ba27d3d3, the rules were changed to add
OPTIONS="string_escape=replace" to creation of
ENV{ID_SERIAL}="$env{ID_MODEL}_$env{ID_SERIAL_SHORT}", so that "/" would be
escaped. But this also changes how the symlink looks for devices that do not
have "/". This adds back the old symlink for compat, except when a slash
is present.

In the meantime, we changed the symlink format to include ${ND_NSID}. Since
the symlink with unescaped characters are older than that, for compat we
only need to cover the older type. (Symlinks without escaping and with ${ND_NSID}
were never created.)  This makes it slightly easier on users: the non-deprecated
symlinks are with "_${ND_NSID}", so they are easier to distinguish.

Fixes #27155.

Mostly untested :(  I only have a boring nvme device with no special characters
in the id, and the symlinks are unchanged for it by this patch.