Create a non-blocking version of BUF_REFCNT() called BUF_REFCNTNB() to be

Create a non-blocking version of BUF_REFCNT() called BUF_REFCNTNB() to be
used for non-critical KASSERT()'s or in situations where the buffer lock
is in a known state.

This fixes a blocking condition in the ATA interrupt path.  The normal
BUF_REFCNT() calls lockcount() which obtains a token which caused the
interrupt thread to temporarily block in biodone() due to a KASSERT.

Found-from: kernel core provided by David Rhodus.