- Cleanup unlocked accesses to buf flags by introducing a new b_vflag member

 - Cleanup unlocked accesses to buf flags by introducing a new b_vflag member
   that is protected by the vnode lock.
 - Move B_SCANNED into b_vflags and call it BV_SCANNED.
 - Create a vop_stdfsync() modeled after spec's sync.
 - Replace spec_fsync, msdos_fsync, and hpfs_fsync with the stdfsync and some
   fs specific processing.  This gives all of these filesystems proper
   behavior wrt MNT_WAIT/NOWAIT and the use of the B_SCANNED flag.
 - Annotate the locking in buf.h