meta-flow: Correctly byteswap skb_priority/skb_mark for mf_value.

meta-flow: Correctly byteswap skb_priority/skb_mark for mf_value.

Fixes the following sparse warnings:

meta-flow.c:947:21: warning: incorrect type in assignment (different base types)
meta-flow.c:947:21:    expected restricted __be32 [usertype] be32
meta-flow.c:947:21:    got unsigned int const [unsigned] [usertype] skb_priority
meta-flow.c:951:21: warning: incorrect type in assignment (different base types)
meta-flow.c:951:21:    expected restricted __be32 [usertype] be32
meta-flow.c:951:21:    got unsigned int const [unsigned] [usertype] skb_mark

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