Correct number of bytes to allocated for slaves in bundle action.

The size of each slave is a uint16_t.  This means that each slave needs 2 bytes
at the end of nx_action_bundle.  Earlier, the size of each slave was not being
factored in when allocating space.  This commit corrects that by allocating 2
bytes for each slave when calculating the total number of bytes to be allocated
at the end of nx_action_bundle.

Signed-off-by: Mehak Mahajan <mmahajan@nicira.com>