sg: Clean up handling of big endian data fields

sg: Clean up handling of big endian data fields

Getting and setting values in SCSI commands and descriptors,
which are big endian, in SG driver can use a bit of cleanup.
This patch simplifies SG driver code by introducing a set of
accessor functions for reading raw big endian values from SCSI
buffers and another set for properly storing the local values
as big endian byte sequences.

The patch also adds some missing endianness conversion macros
in os.h.

Signed-off-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>