Udev rules: remove zvol compat symlinks (without the leading zvol/)

NOTE: this version of the patch is based on top of the commits added in
PR openzfs#12302, which apply fixes to the exact same line of the udev rules
file. That PR is probably best understood as a prerequisite to this one;
but it's not merged currently, so that's why this exists.

This is a potentially arguable change, because it removes some
compatibility cruft that certain systems or people may have come to rely
on (either a very long time ago, or unwisely in recent times).

On the other hand, it's been literally over a decade since OpenZFS
switched to the strategy of using opaque numbered /dev/zd* device nodes,
with the canonical zvol access path being a directory tree of symlinks
created by udev rules inside /dev/zvol/*. (See openzfs#102.) Even at the time,
the /dev/* scheme was labeled as being for "compatibility".

This commit removes the second tree of symlinks located directly at
/dev/*, under the assumption that anybody with any sense has been using
the intended /dev/zvol/* path for a very very long time now.

(The more I think about this, the more I anticipate that some large
fraction of people will have been blissfully unaware that the intention
has been for them to use the /dev/zvol/* tree all along, and they will
have come to rely upon the /dev/* tree simply because it's been there
this whole time despite being a compat thing.)

Signed-off-by: Justin Gottula <justin@jgottula.com>

udev/rules.d/60-zvol.rules.in

@@ -1,6 +1,5 @@
 # Persistent links for zvol
 #
 # persistent disk links: /dev/zvol/dataset_name
-# also creates compatibility symlink of /dev/dataset_name
 
-KERNEL=="zd*", SUBSYSTEM=="block", ACTION=="add|change", PROGRAM=="@udevdir@/zvol_id $devnode", SYMLINK+="zvol/%c %c"
+KERNEL=="zd*", SUBSYSTEM=="block", ACTION=="add|change", PROGRAM=="@udevdir@/zvol_id $devnode", SYMLINK+="zvol/%c"