adduser: create dataset only if home is directly within dataset

Currently, if the prefix of the new home directory is a subdirectory
of a ZFS dataset, adduser will create a new dataset up one or more
levels from the intended destination.  "pw useradd" will then create
a normal directory in the desired location, leaving an unused dataset.
Check for this situation when determining whether to create a dataset,
and let pw create the directory.

Reviewed by:	des
Differential Revision:	https://reviews.freebsd.org/D45229
MFC after:	3 days

usr.sbin/adduser/adduser.sh

@@ -474,7 +474,7 @@ get_homeperm() {
 #	so, enable ZFS home dataset creation.
 #
 get_zfs_home() {
-	local _prefix=
+	local _prefix= _tmp=
 
 	# check if zfs kernel module is loaded before attempting to run zfs to
 	# prevent loading the kernel module on systems that don't use ZFS
@@ -487,6 +487,13 @@ get_zfs_home() {
 		Zcreate="no"
 		return
 	fi
+	# Make sure that _prefix is not a subdirectory within a dataset.  If it
+	# is, the containing dataset will be the same for it and its parent.
+	_tmp=$(${ZFSCMD} list -Ho name "$(dirname "${homeprefix}")" 2>/dev/null)
+	if [ "${_tmp}" = "${_prefix}" ]; then
+		Zcreate="no"
+		return
+	fi
 	zhome="${_prefix}/${username}"
 }