zfs: enable vn_io_fault support

Note that now we have to account for possible partial writes
in dmu_write_uio_dbuf(). It seems that on illumos either all or none
of the data are expected to be written. But the partial writes are
quite expected when vn_io_fault support is enabled.

Reviewed by: kib
MFC after: 7 weeks
Differential Revision: https://reviews.freebsd.org/D2790