Add an argument to vfs_add_vnodeops() to specify VVF_* flags for the vop_ops

Add an argument to vfs_add_vnodeops() to specify VVF_* flags for the vop_ops
structure.  Add a new flag called VVF_SUPPORTS_FSMID to indicate filesystems
which support persistent storage of FSMIDs.  Rework the FSMID code a bit
to reduce overhead.

Use the spare field in the UFS inode structure to implement a persistent
FSMID.  The FSMID is recursively marked in the namecache but not adjusted
until the next getattr() call on the related inode(s), or when the vnode
is reclaimed.