The get_unused_fd() macro was removed upstream by
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=f938612dd9

Thus replace get_unused_fd() by get_unused_fd_flags(0) which used to
be the definition of the get_unused_fd() macro before it was removed.

This fixes cryptodev compilation on kernel 4.0.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>