Move vfs_stall_barrier() from the fd layer into vn_lock() and the vfs layer.

Move vfs_stall_barrier() from the fd layer into vn_lock() and the vfs layer.
vfs stalling is used by suspend/resume and by vmt(4) to stall any
filesystem operation from altering the state on disk. All these
operations will call vn_lock and be stalled. Adjust vfs_stall_barrier()
to allow the lock owner to still progress so that suspend can sync
the filesystems after stalling vfs operation.
OK mpi@