initramfs: Set elevator=noop on the rpool's disks

ZFS already sets elevator=noop for wholedisk vdevs (for all pools), but
typical root-on-ZFS installations use partitions.  This sets
elevator=noop on the disks in the root pool.

Ubuntu 16.04 and 16.10 had this.  It was lost in 17.04 due to Debian
switching to this upstream initramfs script.

Signed-off-by: Richard Laager <rlaager@wiktel.com>

contrib/initramfs/scripts/zfs

@@ -883,6 +883,24 @@ mountroot()
 		/bin/sh
 	fi
 
+	# Set elevator=noop on the root pool's vdevs' disks.  ZFS already
+	# does this for wholedisk vdevs (for all pools), so this is only
+	# important for partitions.
+	"${ZPOOL}" status -L "${ZFS_RPOOL}" 2> /dev/null |
+	    awk '/^\t / && !/(mirror|raidz)/ {
+	        dev=$1;
+	        sub(/[0-9]+$/, "", dev);
+	        print dev
+	    }' |
+	    while read i
+	do
+		if [ -e "/sys/block/$i/queue/scheduler" ]
+		then
+			echo noop > "/sys/block/$i/queue/scheduler"
+		fi
+	done
+
+
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