Support bin-sbin merge on Fedora

Starting with Fedora 42, Fedora has been working on merging /bin and
/sbin directories. See
https://fedoraproject.org/wiki/Changes/Unify_bin_and_sbin

To support this, make sure we do not put files into */sbin directories
on these distributions by respecting the distribution set value of
%{_sbindir}.

In addition, explicitly set `mounthelperdir`, which affects the
placement of `mount.zfs`, which does not respect %{_sbindir} by
default. Making it point to %{_sbindir} will allow it to work correctly
on Fedora, while keeping its previous directory of `/sbin` on all other
distributions.

Note that files that used to reside in */sbin directories on Fedora will
stay accessible under these paths, as the distribution maintains
symlinks. No changes are needed to external scripts invoking these
binaries.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ralf Ertzinger <ralf@skytale.net>
Closes #18373

rpm/generic/zfs.spec.in

@@ -1,4 +1,9 @@
+# Since Fedora 42, Fedora has unified /bin and /sbin. For this to
+# work, we must not override _sbindir
+%if 0%{?fedora} >= 42
+%else
 %global _sbindir    /sbin
+%endif
 %global _libdir     /%{_lib}
 
 # Set the default udev directory based on distribution.
@@ -393,6 +398,7 @@ support for unlocking datasets on user login.
     --with-pammoduledir=%{_libdir}/security \
     --with-python=%{__python} \
     --with-pkgconfigdir=%{_pkgconfigdir} \
+    --with-mounthelperdir=%{_sbindir} \
     --disable-static \
     %{debug} \
     %{debuginfo} \