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

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