smb: client: restore the data_offset bound in is_valid_oplock_break()

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

smb: client: restore the data_offset bound in is_valid_oplock_break()

Commit 83bfbd0bb902 ("cifs: Remove the RFC1002 header from smb_hdr")
changed the quantity this bound is measured against. It used to be
srv->total_read minus the 4-byte RFC1002 preamble that total_read then
included, so it was the SMB message length. The same commit stopped
counting the preamble, and the mechanical substitution to
srv->total_read - srv->pdu_size left an expression that is identically
zero: standard_receive3() reads MID_HEADER_SIZE() bytes and then exactly
pdu_length - MID_HEADER_SIZE() more, adding both to total_read.

len is therefore 0, the subtraction below it wraps, and no __u32
DataOffset can exceed the result, so the check from commit 097f5863b1a0
("cifs: read overflow in is_valid_oplock_break()") no longer rejects
anything. Use total_read, which is now the message length on its own.