ofp-actions: Only set defined bits when encoding "load" actions.

Commit 7eb4b1f1d70345f ("ofp-actions: Support OF1.5 (draft) masked
Set-Field, merge with reg_load.") introduced a bug in that a set_field
action that set an entire field would be translated incorrectly to
reg_load, if the field being set only occupied a portion of the bytes that
it contains.  For example, an MPLS label is 20 bits but has a 4-byte field,
which meant that a set_field would get translated into a reg_load that
wrote all 32 bits; in turn, the receiver of that reg_load would reject it
because it was attempting to set invalid bits (the top 12 bits).

This commit fixes the problem by omitting invalid bits when encoding a
reg_load action.

Reported-by: Pravin Shelar <pshelar@nicira.com>
Tested-by: Pravin Shelar <pshelar@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>