files: drop O_TMPFILE from file descriptor flags

Unnamed temporary files are restored as ghost files.

If O_TMPFILE is set for the open() syscall, the pathname argument
specifies a directory, but criu gives a path to a ghost file.

(00.107450)     36: Error (criu/files-reg.c:1757): Can't open file tmp/#42274874 on restore: Not a directory

Reviewed-by: Dmitry Safonov <0x7f454c46@gmail.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>