Remove VOP_BWRITE(). This function provided a way for a VFS to override

Remove VOP_BWRITE().  This function provided a way for a VFS to override
the bwrite() function and was used *only* by NFS in order to allow NFS to
handle the B_NEEDCOMMIT flag as part of NFSv3's 2-phase commit operation.
However, over time, the handling of this flag was moved to the strategy code.
Additionally, the kernel now fully supports the redirtying of buffers
during an I/O (which both softupdates and NFS need to be able to do).

The override is no longer needed.  All former calls to VOP_BWRITE() now
simply call bwrite().