The problem is that zfs_get_data() supplies a stale zgd_bp to dmu_sync(), which
we then nopwrite against.
zfs_get_data() doesn't hold any DMU-related locks, so after it copies db_blkptr
to zgd_bp, dbuf_write_ready()
could change db_blkptr, and dbuf_write_done() could remove the dirty record.
dmu_sync() then sees the stale
BP and that the dbuf it not dirty, so it is eligible for nop-writing.
The fix is for dmu_sync() to copy db_blkptr to zgd_bp after acquiring the
db_mtx. We could still see a stale
db_blkptr, but if it is stale then the dirty record will still exist and thus
we won't attempt to nopwrite.