pbuf_ctor(): Stop using LK_NOWAIT, use LK_NOWITNESS

The LK_NOWAIT was added to suppress a witness warning, but LK_NOWITNESS
is more what we mean. This makes pbuf_ctor() more consistent with
buf_alloc(), although, unlike buf_alloc(), for pbuf there should not be
any danger of a wild locker relying on the type stability of the buf to
attempt a lock. That is, this is essentially cosmetic.

Relevant history:

• 531f8cfea06b Use dedicated lock name for pbufs
• 5875b94c7493 buf_alloc(): lock the buffer with LK_NOWAIT
• c9e023541aef pbuf_ctor(): lock the buffer with LK_NOWAIT
• 1fb00c8f1060 buf_alloc(): Stop using LK_NOWAIT, use LK_NOWITNESS

Reviewed by: rew, kib
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D45360