Fix volume WR_INDIRECT log replay

The portion of the zvol_replay_write() handler responsible for
replaying indirect log records for some reason never existed.
As a result indirect log records were not being correctly replayed.

This went largely unnoticed since the majority of zvol log records
were of the type WR_COPIED or WR_NEED_COPY prior to OpenZFS 7578.

This patch updates zvol_replay_write() to correctly handle these
log records and adds a new test case which verifies volume replay
to prevent any regression.  The existing test case which verified
replay on filesystem was renamed slog_replay_fs.ksh for clarity.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6603 
Closes #6615

tests/zfs-tests/include/libtest.shlib

@@ -140,7 +140,7 @@ function ismounted
 
 			[[ "$1" == "$dir" || "$1" == "$name" ]] && return 0
 		;;
-		ext2)
+		ext*)
 			out=$(df -t $fstype $1 2>/dev/null)
 			return $?
 		;;