Remove zpool altroot from mountpoint when creating datasets.

If `${ZPOOL}` has `altroot` set, ${ZROOTFS} can only be mounted under
`altroot`. This means `${BASEFS}` _must_ begin with `altroot`.

In `poudriere.conf`, the setting would look something like:
`BASEFS=<altroot>/rest/of/path`.

However, `altroot` is always prepended to whatever path is set to
`mountpoint` at the time of dataset creation. What happens is that
`${ZROOTFS}` is actually mounted on `<altroot>${BASEFS}`, while
`poudriere` thinks it's on `${BASEFS}`.

To fix the problem, simply check if `altroot` is set and remove it
from `mountpoint` when creating datasets. Their `mountpoint`s will
then be prepended with `altroot`, which matches `poudriere`'s internal
records.

src/share/poudriere/common.sh

@@ -1719,7 +1719,7 @@ get_data_dir() {
 		# Explicitly set properties for values diverging from top dataset
 		zfs create -p -o atime=off \
 			-o compression=on \
-			-o mountpoint=${BASEFS} \
+			-o mountpoint="$(remove_altroot ${BASEFS})" \
 			${ZPOOL}${ZROOTFS}
 		zfs create ${ZPOOL}${ZROOTFS}/jails
 		zfs create ${ZPOOL}${ZROOTFS}/ports

src/share/poudriere/include/fs.sh

@@ -23,6 +23,16 @@
 # OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 # SUCH DAMAGE.
 
+remove_altroot() {
+	[ $# -ne 1 ] && eargs remove_altroot mountpoint
+	local mountpoint altroot
+	mountpoint="$1"
+	altroot=
+	[ -n "${NO_ZFS}" ] || altroot=$(zpool get -Ho value altroot "${ZPOOL}")
+
+	echo "${mountpoint}" | sed "s,^${altroot},,"
+}
+
 createfs() {
 	[ $# -ne 3 ] && eargs createfs name mnt fs
 	local name mnt fs
@@ -37,7 +47,7 @@ createfs() {
 		if ! zfs create -p \
 			-o compression=on \
 			-o atime=off \
-			-o mountpoint=${mnt} ${fs}; then
+			-o mountpoint="$(remove_altroot ${mnt})" ${fs}; then
 			echo " fail"
 			err 1 "Failed to create FS ${fs}"
 		fi
@@ -294,7 +304,7 @@ clonefs() {
 			zfs_to=${fs}/${name}
 		fi
 
-		zfs clone -o mountpoint=${mnt} \
+		zfs clone -o mountpoint="$(remove_altroot ${mnt})" \
 			-o sync=disabled \
 			-o atime=off \
 			-o compression=off \