Thread might create a condition for delayed SU cleanup, which creates
a reference to the mount point in td_su, but exit without returning
through userret(), e.g. when terminating due to single-threading or
process exit. In this case, td_su reference is not dropped and mount
point cannot be freed.
Handle the situation by clearing td_su also in the thread destructor
and in exit1(). softdep_ast_cleanup() has to receive the thread as
argument, since e.g. thread destructor is executed in different
context.
Reported and tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
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