meta-flow: Avoid unnecessary large memset.

meta-flow: Avoid unnecessary large memset.

mf_mask_field_and_prereqs() used to memset a static variable again and
again.  Now that mf_value is larger (due to tun_metadata field), this
is more expensive.  Avoid this by using static initialization.

mf_mask_field_and_prereqs() is used only for set field and reg move,
which never deal with the tun_metadata field as a whole.

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