Don't return early for non-failure for one of the EMLINK checks.

r326987 enabled two #if 0'd-out EMLINK checks in zfs_link_create() for
link overflow. However, one of the checks (when the vnode adding a link
is a directory such as for mkdir) always returned even if the link did not
overflow. Change this to only return early if it needs to report an
EMLINK error.

Reported by: db, shurd
Sponsored by: Chelsio Communications