SUNRPC: prevent task_cleanup running on freed xprt

We saw a report of a NULL dereference in xprt_autoclose:

	https://bugzilla.redhat.com/show_bug.cgi?id=611938

This appears to be the result of an xprt's task_cleanup running after
the xprt is destroyed.  Nothing in the current code appears to prevent
that.

Signed-off-by: J. Bruce Fields <[email protected]>
Signed-off-by: Trond Myklebust <[email protected]>
1 file changed