Remove an unneeded 'tail -n 1' from a pipeline

When piping to awk, it's almost always an anti-pattern to use 'grep'
first.

When not in a pipeline, sometimes it is faster to use tail, as awk
must process all the lines in the input stream, and won't 'seek'.
In a pipeline, both grep and awk must process all lines, so we might
as well skip the extra process creation for tail and just use awk
for all the processing.

Reviewed by:	jilles
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D19441

libexec/rc/rc.d/growfs

@@ -57,7 +57,7 @@ growfs_start ()
 		;;
 	zfs)
 		pool=${FSDEV%%/*}
-		rootdev=$(zpool list -v $pool | tail -n 1 | awk '{ print $1 }')
+		rootdev=$(zpool list -v $pool | awk 'END { print $1 }')
 		;;
 	*)
 		echo "Don't know how to grow root filesystem type: $FSTYPE"