Change /etc/mtab to /proc/self/mounts

Fix misleading error message:

 "The /dev/zfs device is missing and must be created.", if /etc/mtab is missing.

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Eric Desrochers <eric.desrochers@canonical.com>
Closes #4680 
Closes #5029

etc/init.d/zfs-functions.in

@@ -368,7 +368,7 @@ read_mtab()
 			# Set the variable.
 			eval export MTAB_$mntpnt=\"$fs\"
 		fi
-	done < /proc/mounts
+	done < /proc/self/mounts
 }
 
 in_mtab()

etc/init.d/zfs-mount.in

@@ -39,7 +39,7 @@ chkroot() {
 		if [ "$2" = "/" ]; then
 			return 0
 		fi
-	done < /etc/mtab
+	done < /proc/self/mounts
 
 	return 1
 }
@@ -178,7 +178,7 @@ do_start()
 
 	check_module_loaded "zfs" || exit 0
 
-	# Ensure / exists in /etc/mtab, if not update mtab accordingly.
+	# Ensure / exists in /proc/self/mounts.
 	# This should be handled by rc.sysinit but lets be paranoid.
 	if ! chkroot
 	then