ofproto-dpif-xlate: Refactor xlate_table_action() to avoid Clang warnings.

ofproto-dpif-xlate: Refactor xlate_table_action() to avoid Clang warnings.

I get a bunch of thread-safety warnings with the latest Clang without this
patch, because Clang is smart enough to see locking and unlocking but not
smart enough to figure out the relationships.  This refactoring avoids the
warnings.

I first noticed these warnings with Clang 1:3.4~svn188890-1~exp1.
I previously used version 1:3.4~svn187484-1~exp1.

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