Add minimal reproducer for ZoL issue 4083

Adds a test pool which designed to be a minimal reproducer of
openzfs/zfs#4083.  The pool contains
a single filesystem "tank" with only a file "file2" in it. When we try
to delete the file we trigger an assertion in zfs_remove(): this is
because the file xattr obj was already deleted. This data discrepancy
is not detected by a scrub which completes successfully.

Signed-off-by: loli10K <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Closes #3

Author: loli10K

README.markdown

@@ -73,3 +73,12 @@ zol-0.7.0-rc1-be-ziltest| This pool was created so that we might have a big
 		       | details on this problem. The pool's name is 
 		       | ziltest.26210 and it contains the "ziltest.26210/fs"
 		       | filesystem made by ziltest.sh.
+		       |
+zol-znode-removed-xattr| This pool was created to be a minimal reproducer of
+                       | https://github.com/zfsonlinux/zfs/issues/4083.
+                       | The pool contains a single filesystem "tank" with only
+                       | a file "file2" in it. When we try to delete the file we
+                       | trigger an assertion in zfs_remove(): this is because
+                       | the file xattr obj was already deleted. This data
+                       | discrepancy is not detected by a scrub which completes
+                       | successfully.