smb: client: fix request buffer leak in smb2_new_read_req()

In the Linux kernel, the following vulnerability has been resolved:

smb: client: fix request buffer leak in smb2_new_read_req()

smb2_new_read_req() allocates the request buffer with
smb2_plain_req_init() but only publishes it to the caller with
*buf = req at the very end of the function. Two error returns sit in
between:

rc = smb2_plain_req_init(SMB2_READ, io_parms->tcon, server,
(void **) &req, total_len);
if (rc)
return rc;

if (server == NULL)
return -ECONNABORTED;
[...]
rdata->mr = smbd_register_mr(server->smbd_conn,
&rdata->subreq.io_iter,
true, need_invalidate);
if (!rdata->mr)
return -EAGAIN;

On either of them the buffer is neither released nor handed back, so
it is leaked. The caller cannot clean up after it: smb2_async_readv()
does 'goto out' on a non-zero return, which skips the
cifs_small_buf_release(buf) at async_readv_out, and buf has not been
assigned at that point in any case.

The write path has never had this problem. smb2_async_writev()
registers the memory region inline and jumps to its release label
instead of returning:

wdata->mr = smbd_register_mr(...);
if (!wdata->mr) {
rc = -EAGAIN;
goto async_writev_out;
}

Commit b7972092199f ("cifs: smbd: Retry on memory registration
failure") changed both sides from -ENOBUFS to -EAGAIN in a single
patch, which puts the two shapes next to each other.

Only the -EAGAIN return is reachable in practice, because
smb2_plain_req_init() calls smb2_reconnect() first and that already
fails with -EIO when server is NULL, before anything is allocated.
Both returns are given the same treatment here rather than leaving
one of them correct only by accident.

Because -EAGAIN is a replayable error, the failure also reaches the
retry block at the end of smb2_async_readv(), which marks the
subrequest NETFS_SREQ_NEED_RETRY, so a failing registration can be
retried rather than ending the I/O, and every attempt that reaches it
leaks another buffer. smb2_should_replay() short-circuits on
tcon->retry, so on a hard mount the attempt count is not bounded by
the retrans setting.

Only the asynchronous read path is affected. The synchronous
SMB2_read() caller passes rdata == NULL and the memory registration
block is guarded on rdata.

The memory registration failure path was pointed out by the Sashiko
AI reviewer while it was reviewing an unrelated patch to
smb2_async_readv().