meta-flow: Fix and simplify mf_get_mask().

meta-flow: Fix and simplify mf_get_mask().

This function can be implemented as a trivial wrapper around
mf_get_value(), which I hadn't noticed before, so it's better to do it
that way.  Also, examining the code that is removed, it had some bugs in
it (for example, all MFF_TUN_* fields were treated as if they were
MFF_TUN_ID) which mf_get_value() does not have, so this fixes bugs too.

Signed-off-by: Ben Pfaff <blp@nicira.com>