smbfs allocates rq_trans2buffer to handle server's multi transaction2 response
messages.  As struct smb_request may be reused, rq_trans2buffer is freed
before each new request.  However if last servers's response is not multi but
single trans2 message then new rq_trans2buffer is not allocated but last
smb_rput still tries to free it again.

To prevent this issue rq_trans2buffer pointer should be set to NULL after
kfree.
Signed-off-by: Vasily Averin <vvs@sw.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>