MFV r254749:

MFV r254749:

Don't hold dd_lock for long by breaking it when not doing dsl_dir
accounting.  It is not necessary to hold the lock while manipulating
the parent's accounting, because there is no interface for userland
to see a consistent picture of both parent and child at the same
time anyway.

Illumos ZFS issues:
  4046 dsl_dataset_t ds_dir->dd_lock is highly contended