libnv: Don't lose big-endian flag when receiving a message.

A bug caused the "big endian" flag to be lost when receiving a message. As a
result, the bits are interpreted as little endian and an extremely large
allocation is attempted.

This change fixes ping(8)'s communication to casperd(8) on big-endian
architectures.

Reported by:	Anton Shterenlikht
Tested by:	danfe
Get rid of ICL lock; use upper-layer (initiator or target) lock instead.
This avoids extra locking in icl_pdu_queue(); the upper layer needs to call
it while holding its own lock anyway, to avoid sending PDUs out of order.

Sponsored by:	The FreeBSD Foundation