smb: client: reject out-of-bounds DataOffset in CIFSSMBRead()

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

smb: client: reject out-of-bounds DataOffset in CIFSSMBRead()

The SMB1 synchronous read helper CIFSSMBRead() validates the server's
DataLength against CIFSMaxBufSize and the caller's count, but never
validates DataOffset. The copy source is formed as

&pSMBr->hdr.Protocol + le16_to_cpu(pSMBr->DataOffset)

and memcpy()'d for DataLength bytes with no check that the
[DataOffset, DataOffset + DataLength) range lies within the response
actually received from the server.

A malicious or compromised SMB1 server can return a response carrying
an in-range DataLength and a large DataOffset, driving the source
pointer past the end of the response buffer. The memcpy() then copies
adjacent kernel heap into the caller's read buffer (information
disclosure), or reads unmapped memory and oopses (denial of service).
SMB1 is not negotiated by default; reaching this code requires an
explicit vers=1.0 mount.

Both DataOffset and the received response length recorded in
rsp_iov.iov_len are relative to the start of the SMB header, so reject
the response unless DataOffset + DataLength fits within that length,
using overflow-safe arithmetic, before forming the source pointer.
The response length has been validated by the previous patch, so the
DataOffset and DataLength fields can be read safely here.

While here, make data_length unsigned. It holds a length derived from
unsigned on-the-wire fields and is only ever compared against unsigned
quantities; print it with %u accordingly, and add __func__ to the
cifs_dbg() calls in this function.