iohelper: use saferead if later write with O_DIRECT

iohelper: use saferead if later write with O_DIRECT

One of the usecases of iohelper is to read from pipe and write
to file with O_DIRECT. As we read from pipe we can have partial
read and then we fail to write this data because output file
is open with O_DIRECT and buffer size is not aligned.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>