meta-flow: Simplify handling of a variable number of registers.

At the time that Open vSwitch implemented registers, there was a high cost
to adding additional fields, so I wrote the code so that the number of
registers could be reduced at compile time.  Now, fields are cheaper
(though not free) and in the meantime I have never heard of anyone reducing
the number of registers.  Since I intend to add more code that would
require awkward "#if"s like this, I think that this is a good time to
simplify it by requiring FLOW_N_REGS to be fixed.  This commit does that.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>