rsocket: Reserve bits in data transfer message

rsockets includes the length of a data transfer in the
immediate data.  Currently 29 bits are reserved for the
length, but the maximum transfer is currently capped
at 64K.  For future expansion, limit the length field
to 17 bits and reserve the other 12.

The interpretation of the 17 bits is modified as follows.
The lower 16 bits indicate a length.  If the upper bit is
0, then the lower 16 bits indicate a transfer size in bytes.
If the upper bit is 1, the the total transfer size is
calculated as (size carried in bits 0-15 + 1) * 64 KB.

This is backwards compatible with the current implementation,
allows for transfers up to 4 GB, and provides us some
additional bits for future use.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>