The trinity fuzzer calls pwritev with an iovec that has one or more entries
which point to some initial valid data and then the rest point to addresses
which are not mapped. This yields EFAULT once the write hits the invalid
address, but we do successfully complete some amount of writing. The zfs_write
code does not handle this properly. It loses track of the error return from
dmu_write_uio_dbuf and it has an invalid ASSERT which does not account for the
partial write case.