smbfs: fix calculation of kernel_recvmsg size parameter in smb_receive()

smb_receive calls kernel_recvmsg with a size that's the minimum of the
amount of buffer space in the kvec passed in or req->rq_rlen (which
represents the length of the response).  This does not take into account
any data that was read in a request earlier pass through smb_receive.

If the first pass through smb_receive receives some but not all of the
response, then the next pass can call kernel_recvmsg with a size field
that's too big.  kernel_recvmsg can overrun into the next response,
throwing off the alignment and making it unrecognizable.

This causes messages like this to pop up in the ring buffer:

smb_get_length: Invalid NBT packet, code=69

as well as other errors indicating that the response is unrecognizable.
Typically this is seen on a smbfs mount under heavy I/O.

This patch changes the code to use (req->rq_rlen - req->rq_bytes_recvd)
instead instead of just req->rq_rlen, since that should represent the
amount of unread data in the response.

I think this is correct, but an ACK or NACK from someone more familiar
with this code would be appreciated...

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>