pbuf_ctor(): lock the buffer with LK_NOWAIT

This LOR happens when reading from a file backed MD device:

lock order reversal:
 1st 0xfffffe00431eaac0 pbufwait (pbufwait, lockmgr) @ /cobra/src/sys/vm/vm_pager.c:471
 2nd 0xfffff80003f17930 ufs (ufs, lockmgr) @ /cobra/src/sys/dev/md/md.c:977
lock order pbufwait -> ufs attempted at:
    #0 0xffffffff80c78ead at witness_checkorder+0xbdd
    #1 0xffffffff80bd6a52 at lockmgr_lock_flags+0x182
    #2 0xffffffff80f52d5c at ffs_lock+0x6c
    #3 0xffffffff80d0f3f4 at _vn_lock+0x54
    #4 0xffffffff80708629 at mdstart_vnode+0x499
    #5 0xffffffff807060ec at md_kthread+0x20c
    #6 0xffffffff80bbfcd0 at fork_exit+0x80
    #7 0xffffffff810b809e at fork_trampoline+0xe

This LOR was previously blessed by witness before commit 531f8cfea0
("Use dedicated lock name for pbufs").

Instead of blessing ufs and pbufwait, use LK_NOWAIT to prevent recording
the lock order. LK_NOWAIT will be a nop here as the lock is dropped in
pbuf_dtor(). The takes the same approach as 5875b94c74 ("buf_alloc():
lock the buffer with LK_NOWAIT").

Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D34183

sys/vm/vm_pager.c

@@ -467,7 +467,7 @@ pbuf_ctor(void *mem, int size, void *arg, int flags)
 	bp->b_ioflags = 0;
 	bp->b_iodone = NULL;
 	bp->b_error = 0;
-	BUF_LOCK(bp, LK_EXCLUSIVE, NULL);
+	BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT, NULL);
 
 	return (0);
 }