9p: statfs should not override server f_type

Allow a 9p2000.L server to supply the statfs f_type value rather than
hardwiring V9FS_MAGIC.  It is desirable to give the server this option
in some applications, e.g. I/O forwarding.

Signed-off-by: Jim Garlick <garlick@llnl.gov>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>