Fix unlinked file cannot do xattr operations

Currently, doing things like fsetxattr(2) on an unlinked file will result in
ENODATA. There's two places that cause this: zfs_dirent_lock and zfs_zget.

The fix in zfs_dirent_lock is pretty straightforward. In zfs_zget though, we
need it to not return error when the zp is unlinked. This is a pretty big
change in behavior, but skimming through all the callers, I don't think this
change would cause any problem. Also there's nothing preventing z_unlinked
from being set after the z_lock mutex is dropped before but before zfs_zget
returns anyway.

The rest of the stuff is to make sure we don't log xattr stuff when owner is
unlinked.

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>

include/sys/zfs_znode.h

@@ -194,6 +194,7 @@ typedef struct znode {
 	zfs_acl_t	*z_acl_cached;	/* cached acl */
 	krwlock_t	z_xattr_lock;	/* xattr data lock */
 	nvlist_t	*z_xattr_cached; /* cached xattrs */
+	uint64_t	z_xattr_parent;	/* parent obj for this xattr */
 	list_node_t	z_link_node;	/* all znodes in fs link */
 	sa_handle_t	*z_sa_hdl;	/* handle to sa data */
 	boolean_t	z_is_sa;	/* are we native sa? */