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