[PKT_SCHED] CLS_U32: Use ffs() instead of C code on hash mask to get first set bit.

Computing the rank of the first set bit in the hash mask (for using later
in u32_hash_fold()) was done with plain C code. Using ffs() instead makes
the code more readable and improves performance (since ffs() is better
optimized in assembler).

Using the conditional operator on hash mask before applying ntohl() also
saves one ntohl() call if mask is 0.

Signed-off-by: Radu Rendec <radu.rendec@ines.ro>
Signed-off-by: Jarek Poplawski <jarkao2@o2.pl>
Acked-by: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>