Some tests expect that all the data will be handled in a single invocation
of read/write system call. But it possible to have short read/write on a
loaded system and this is not an error.
Add helper functions that will reliably read/write the entire buffer.
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>