improve tomb umount checks avoiding grep call

consolidate use of `list_tomb_mounts` also inside umount_tomb
this should fix #315

tomb

@@ -2942,22 +2942,18 @@ umount_tomb() {
 		# posing a security risk.
 		# See https://github.com/dyne/Tomb/issues/273
 
-		# checking for tombs
-		mount | grep -w "$tombmount" >/dev/null
-		mount_status=$?
-		# return value of 0 for grep means it found at least one entry
-		# return value of 1 means nothing was found, implying, the tomb
-		# mount was already umounted.
-		if [ $mount_status = 0 ]; then
-			# Tomb was not umounted through the above command
-			# Will do so now
-			_verbose "Performing umount of ::1 mount point::" $tombmount
-			_sudo umount ${tombmount}
-			[[ $? = 0 ]] || { _failure "Tomb is busy, cannot umount!" }
-		else
-			# Tomb was already umounted, will not do it again
-			_warning "Tomb was already umounted, possibly through a binded directory"
-		fi
+		# checking for tombs still mounted
+		mounted_tombs=(`list_tomb_mounts`)
+		for t in ${mounted_tombs}; do
+			usedmount=${t[(ws:;:)2]}
+			[[ "$usedmount" == "$tombmount" ]] && {
+				# Tomb was not umounted through the above command
+				# Will do so now
+				_verbose "Performing umount of ::1 mount point::" $tombmount
+				_sudo umount ${tombmount}
+				[[ $? = 0 ]] || { _failure "Tomb is busy, cannot umount!" }
+			}
+		done
 
 		# If we used a default mountpoint and is now empty, delete it
 		tombname_regex=${tombname//\[/}