d_strategy: Perform I/O for internal functions

This type of function is called by the file system code when input/output to the physical device is to be performed. It is normally defined only for block special devices, but can also be defined by character special devices. There is no external interface to the file system that allows a user to use the strategy function. A prime example of the use of this function is z/TPF system database block special device driver cbdtpf.cpp. It is used by both the fixed file system (FFS) and pool file system (PFS) for database access.