Update zfs_destroy_004.ksh script

Update zfs_destroy_004.ksh script

Issues:
Under Linux, when executing zfs_destroy_004.ksh destroy $fs is an
error.  The key issue here is that illumos kernel treats this case
differently than the Linux kernel. On illumos you can unmount and
destroy a filesystem which is busy and all consumers of it get EIO.
On Linux the expected behavior is to prevent the unmount and destroy.

Cause analysis:
When create $fs file system and mount file system to $mntp.
cd $mntp, linux isn't allow to destroy $fs in this mount contents.
No matter what destroy with parameters.

Solution:
So  log_mustnot $ZFS destroy $fs is ok.
cd $olddir and destroy $fs.

Signed-off-by: caoxuewen cao.xuewen@zte.com.cn
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #5012