rsocket: Dedicate a fixed number of SQEs for control messages

The number of SQEs allocated for control messages is set
to 1 of 2 constant values (either 4 or 2).  A default
value is used unless the size of the SQ is below a certain
threshold (16 entries).  This results in additional code
complexity, and it is highly unlikely that the SQ would
ever be allocated smaller than 16 entries.

Simplify the code to use a single constant value for the
number of SQEs allocated for control messages.  This will
also help in subsequent patches that will need to deal
with HCAs that do not support inline data.