MFC r189878:
Fix two issues with bufdaemon, often causing the processes to hang in
the "nbufkv" sleep.

Do not retry request for the new block from ffs_bufwrite() when write is done
from bufdaemon and there is a buffer shortage.

In getnewbuf(), help bufdaemon to flush dirty buffers owned by the vnode
locked by curthread.

For MFC, default value for sysctl vfs.flushbufqtarget is set to -1,
disabling the helpers. The TDP_BUFNEED flag value from HEAD conflicts
with TDP_UPCALLING KSE bit, so it is moved to the end of allocated bits.