queueing up buffers and awakening the MFS server process to do the I/O,
we do the I/O to the server process's address space directly using
facilities provided by UVM.
This makes it possible for buffers attempting to flush out while the
MFS is being unmounted to actually do the I/O, where before it would
fail if the server process wasn't in the MFS idle loop (i.e. had been
signaled and was attempting to exit).
Should fix kern/10122 (I can no longer reproduce the problem described
in the PR when running with these changes), and any number of other
MFS-related complaints made by people over time.
