fstrm_io: wrap the writer 'write_data' and 'write_control' method calls
fstrm_io: wrap the writer 'write_data' and 'write_control' method calls

this improves readability very slightly and makes sure that error
handling is always done: if a 'write_data' or 'write_control' method
call fails we need to always mark the writer non-writable so that a
re-open is scheduled.

farsightsec/fstrm,farsightsec/fstrm