When dmu_tx_assign is called from zil_lwb_write_issue, it's possible
for either ERESTART or EIO to be returned.
If ERESTART is returned, this will cause an assertion to fail directly
in zil_lwb_write_issue, where the code assumes the return value is
EIO if dmu_tx_assign returns a non-zero value. This can occur if the
SPA is suspended when dmu_tx_assign is called, and most often occurs
when running zloop.
If EIO is returned, this can cause assertions to fail elsewhere in the
ZIL code. For example, zil_commit_waiter_timeout contains the
following logic:
  lwb_t *nlwb = zil_lwb_write_issue(zilog, lwb);
  ASSERT3S(lwb->lwb_state, !=, LWB_STATE_OPENED);
In this case, if dmu_tx_assign returned EIO from within
zil_lwb_write_issue, the lwb variable passed in will not be issued
to disk. Thus, it's lwb_state field will remain LWB_STATE_OPENED and
this assertion will fail. zil_commit_waiter_timeout assumes that after
it calls zil_lwb_write_issue, the lwb will be issued to disk, and
doesn't handle the case where this is not true; i.e. it doesn't handle
the case where dmu_tx_assign returns EIO.