OpenZFS 9329 - panic in zap_leaf_lookup() due to concurrent zapification

For the null pointer issue shown below, the solution is to initialize the
contents of the object before changing its type, so that concurrent accessors
will see it as non-zapified until it is ready for access via the ZAP.

    BAD TRAP: type=e (#pf Page fault) rp=ffffff00ff520440 addr=20 occurred
    in module "zfs" due to a NULL pointer dereference

    ffffff00ff520320 unix:die+df ()
    ffffff00ff520430 unix:trap+dc0 ()
    ffffff00ff520440 unix:cmntrap+e6 ()
    ffffff00ff520590 zfs:zap_leaf_lookup+46 ()
    ffffff00ff520640 zfs:fzap_lookup+a9 ()
    ffffff00ff5206e0 zfs:zap_lookup_norm+111 ()
    ffffff00ff520730 zfs:zap_contains+42 ()
    ffffff00ff520760 zfs:dsl_dataset_has_resume_receive_state+47 ()
    ffffff00ff520900 zfs:get_receive_resume_stats+3e ()
    ffffff00ff520a90 zfs:dsl_dataset_stats+262 ()
    ffffff00ff520ac0 zfs:dmu_objset_stats+2b ()
    ffffff00ff520b10 zfs:zfs_ioc_objset_stats_impl+64 ()
    ffffff00ff520b60 zfs:zfs_ioc_objset_stats+33 ()
    ffffff00ff520bd0 zfs:zfs_ioc_dataset_list_next+140 ()
    ffffff00ff520c80 zfs:zfsdev_ioctl+4d7 ()
    ffffff00ff520cc0 genunix:cdev_ioctl+39 ()
    ffffff00ff520d10 specfs:spec_ioctl+60 ()
    ffffff00ff520da0 genunix:fop_ioctl+55 ()
    ffffff00ff520ec0 genunix:ioctl+9b ()
    ffffff00ff520f10 unix:brand_sys_sysenter+1c9 ()

Porting Notes:
* DMU_OT_BYTESWAP conditional in zap_lockdir_impl() kept.

Authored by: Matthew Ahrens <[email protected]>
Reviewed by: Pavel Zakharov <[email protected]>
Reviewed by: Brad Lewis <[email protected]>
Reviewed-by: George Melikov <[email protected]>
Approved by: Dan McDonald <[email protected]>
Ported-by: Brian Behlendorf <[email protected]>

OpenZFS-issue: https://illumos.org/issues/9329
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/e8e0f97
Closes #7578

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