smbmsg: use a more convenient way of accessing data read from a slave

Developers writing code for accessing /dev/smb may use this base utility
as an example. Now that SMB_READB, SMB_READW, SMB_PCALL behave as
documented, wwe can use them in a more convenient way than before.

MFC after: 4 weeks
X-MFC after: r308527