Reading and writing large buffers may result in short read/write. In cases
we expect the entire buffer to be transferred use {read,write}_data rather
than plain read/write syscalls.

Reported-by: Mr Jenkins
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>