example/passthrough_hp: No auto FOPEN_DIRECT_IO in passthrough mode

example/passthrough_hp: No auto FOPEN_DIRECT_IO in passthrough mode

sfs_open and sfs_create set fi->direct_io (FOPEN_DIRECT_IO) when
O_DIRECT is given, in order to benefit from a shared inode lock
in kernel, i.e. to get parallel DIO writes. However, kernel side
disabled passthrough when FOPEN_DIRECT_IO is set. Reads/writes
had been totally failing in this case for O_DIRECT as
sfs_write_buf() and sfs_read() have a sanity check. That sanity
check could be modified, but for performance passthrough is
better than parallel DIO, hence, we only want automatic
FOPEN_DIRECT_IO for O_DIRECT when passthrough is not enabled.

Fixes: https://github.com/libfuse/libfuse/issues/1027
This also fixes automatically switching to FOPEN_DIRECT_IO
for O_DIRECT in sfs_create().