lib/flow: Optimize minimask_has_extra() and minimask_is_catchall()

lib/flow: Optimize minimask_has_extra() and minimask_is_catchall()

We only need to iterate over the bits masked by the 'b' in
minimask_has_extra(), since for zeroes in 'b' there can be no 'extra'
wildcards in 'a', as 'b' has already wildcarded all the bits.

minimask_is_catchall() can be simplified by the invariant that mask's
map never has 1-bits for all-zero values.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>