fcntl: allow to set O_DIRECT flag on pipe

With packetized mode for pipes, it's not possible to set O_DIRECT on pipe file
via sys_fcntl, because of unsupported sanity checks.
Ability to set this flag will be used by CRIU to migrate packetized pipes.

v2:
Fixed typos and mode variable to check.

Signed-off-by: Stanislav Kinsburskiy <skinsbursky@virtuozzo.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>