smb: client: fix UAF and buffer leak in cifs_check_trans2() for malformed secondary T2

When a valid primary TRANSACT2 response has been received (mid->resp_buf
set, mid->multiRsp true) and a subsequent secondary response causes
cifs_check_trans2() to return false -- either because the SMB header is
invalid (malformed != 0) or because check2ndT2() rejects the PDU --
handle_mid() overwrites mid->resp_buf with the new buffer (leaking the
primary buffer) and, because mid->multiRsp is set, skips the
server->smallbuf/bigbuf NULL-out.  When the user thread frees
mid->resp_buf, server->smallbuf or server->bigbuf is left dangling; the
demux thread reuses it for the next packet, resulting in a use-after-free.

Combine both early-exit conditions and, when mid->multiRsp is already
set, abort the pending transaction inline: set multiEnd, call
dequeue_mid() with malformed=true, and return true so handle_mid() exits
without touching mid->resp_buf or the server buffer pointers.

Fixes: 316cf94a910f ("CIFS: Move trans2 processing to ops struct")
Cc: stable@vger.kernel.org # cifs_check_trans2() is in smb1ops.c on kernels < 7.0
Signed-off-by: Frank Sorenson <sorenson@redhat.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>