Clean up by-dnode code in dmu_tx.c

https://github.com/zfsonlinux/zfs/commit/0eef1bde31d67091d3deed23fe2394f5a8bf2276
introduced some changes which we slightly improved the style of when
porting to illumos.

There is also one minor error-handling fix, in zap_add() the "zap" may
become NULL in case of an error re-opening the ZAP.

Originally suggested at: https://github.com/openzfs/openzfs/pull/276

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #5805