Illumos 4039 - zfs_rename()/zfs_link() needs stronger test for XDEV

4039 zfs_rename()/zfs_link() needs stronger test for XDEV
Reviewed by: Gordon Ross <gordon.ross@nexenta.com>
Reviewed by: Kevin Crowe <kevin.crowe@nexenta.com>
Reviewed by: Saso Kiselkov <skiselkov.ml@gmail.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@nexenta.com>

References:
  https://www.illumos.org/issues/4039
  illumos/illumos-gate@18e6497

Porting notes:
- This check was updated in Linux in a similar fashion early on in
  the port.  Therefore, this patch just reorders the function and
  updates the comment so it flows the same way as the upstream code.

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4218

Author: mtelka

module/zfs/zfs_vnops.c

@@ -3207,13 +3207,18 @@ zfs_rename(struct inode *sdip, char *snm, struct inode *tdip, char *tnm,
 	ZFS_VERIFY_ZP(sdzp);
 	zilog = zsb->z_log;
 
+	tdzp = ITOZ(tdip);
+	ZFS_VERIFY_ZP(tdzp);
+
+	/*
+	 * We check i_sb because snapshots and the ctldir must have different
+	 * super blocks.
+	 */
 	if (tdip->i_sb != sdip->i_sb || zfsctl_is_node(tdip)) {
 		ZFS_EXIT(zsb);
 		return (SET_ERROR(EXDEV));
 	}
 
-	tdzp = ITOZ(tdip);
-	ZFS_VERIFY_ZP(tdzp);
 	if (zsb->z_utf8 && u8_validate(tnm,
 	    strlen(tnm), NULL, U8_VALIDATE_ENTIRE, &error) < 0) {
 		ZFS_EXIT(zsb);
@@ -3751,14 +3756,18 @@ zfs_link(struct inode *tdip, struct inode *sip, char *name, cred_t *cr)
 		return (SET_ERROR(EPERM));
 	}
 
+	szp = ITOZ(sip);
+	ZFS_VERIFY_ZP(szp);
+
+	/*
+	 * We check i_sb because snapshots and the ctldir must have different
+	 * super blocks.
+	 */
 	if (sip->i_sb != tdip->i_sb || zfsctl_is_node(sip)) {
 		ZFS_EXIT(zsb);
 		return (SET_ERROR(EXDEV));
 	}
 
-	szp = ITOZ(sip);
-	ZFS_VERIFY_ZP(szp);
-
 	/* Prevent links to .zfs/shares files */
 
 	if ((error = sa_lookup(szp->z_sa_hdl, SA_ZPL_PARENT(zsb),