ZTS: Fix a few defaults

Linux was missing a default value for DEV_DSKDIR. Set it to /dev.
Fix resulting fallout.

SLICE_PREFIX seems like a good candidate for including in the defaults.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #9898

tests/zfs-tests/include/default.cfg.in

@@ -173,6 +173,7 @@ if is_linux; then
 
 	ZVOL_DEVDIR="/dev/zvol"
 	ZVOL_RDEVDIR="/dev/zvol"
+	DEV_DSKDIR="/dev"
 	DEV_RDSKDIR="/dev"
 	DEV_MPATHDIR="/dev/mapper"
 
@@ -182,8 +183,8 @@ if is_linux; then
 	VDEVID_CONF="$ZEDLET_DIR/vdev_id.conf"
 	VDEVID_CONF_ETC="/etc/zfs/vdev_id.conf"
 
-
 	NEWFS_DEFAULT_FS="ext2"
+	SLICE_PREFIX=""
 elif is_freebsd; then
 	unpack_opts="xv"
 	pack_opts="cf"
@@ -198,6 +199,7 @@ elif is_freebsd; then
 	DEV_MPATHDIR="/dev/multipath"
 
 	NEWFS_DEFAULT_FS="ufs"
+	SLICE_PREFIX="p"
 else
 	unpack_opts="xv"
 	pack_opts="cf"
@@ -211,8 +213,9 @@ else
 	DEV_RDSKDIR="/dev/rdsk"
 
 	NEWFS_DEFAULT_FS="ufs"
+	SLICE_PREFIX="s"
 fi
 export unpack_opts pack_opts verbose unpack_preserve pack_preserve \
        ZVOL_DEVDIR ZVOL_RDEVDIR DEV_DSKDIR DEV_RDSKDIR DEV_MPATHDIR \
        ZEDLET_DIR ZED_LOG ZED_DEBUG_LOG VDEVID_CONF VDEVID_CONF_ETC \
-       NEWFS_DEFAULT_FS
+       NEWFS_DEFAULT_FS SLICE_PREFIX