bash_completion.d: always call zfs/zpool binaries directly

/dev/zfs is 0:0 666 on most systems, so the [ -w /dev/zfs ] check always
succeeds, but if zfs isn't in $PATH (e.g. when completing from
"/sbin/zfs list" on a regular account) this can lead to error spew like

  nabijaczleweli@szarotka:~$ /sbin/zfs list bash: zfs: command not found
  @ bash: zfs: command not found

We only do read-only commands, and quite general ones at that,
so there's no need to elevate one way or another.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11828

contrib/bash_completion.d/.gitignore

@@ -0,0 +1 @@
+/zfs

contrib/bash_completion.d/Makefile.am

@@ -1,5 +1,8 @@
+include $(top_srcdir)/config/Substfiles.am
+
 bashcompletiondir = $(sysconfdir)/bash_completion.d
 
 noinst_DATA = zfs
 
-EXTRA_DIST = $(noinst_DATA)
+EXTRA_DIST += $(noinst_DATA)
+SUBSTFILES += $(noinst_DATA)

contrib/bash_completion.d/zfs.in

@@ -21,13 +21,8 @@
 # FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 # OTHER DEALINGS IN THE SOFTWARE.
 
-if [[ -w /dev/zfs ]]; then
-    __ZFS_CMD="zfs"
-    __ZPOOL_CMD="zpool"
-else
-    __ZFS_CMD="sudo zfs"
-    __ZPOOL_CMD="sudo zpool"
-fi
+__ZFS_CMD="@sbindir@/zfs"
+__ZPOOL_CMD="@sbindir@/zpool"
 
 # Disable bash's built-in hostname completion, as this makes it impossible to
 # provide completions containing an @-sign, which is necessary for completing