dn_struct_rwlock can not be held in dmu_tx_try_assign()

The thread calling dmu_tx_try_assign() can't hold the dn_struct_rwlock
while assigning the tx, because this can lead to deadlock. Specifically,
if this dnode is already assigned to an earlier txg, this thread may
need to wait for that txg to sync (the ERESTART case below). The other
thread that has assigned this dnode to an earlier txg prevents this txg
from syncing until its tx can complete (calling dmu_tx_commit()), but it
may need to acquire the dn_struct_rwlock to do so (e.g. via
dmu_buf_hold*()).

This commit adds an assertion to dmu_tx_try_assign() to ensure that this
deadlock is not inadvertently introduced.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #8929