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

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
  https://github.com/illumos/illumos-gate/commit/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