9p: add 9P2000.L statfs operation

I made a V2 of this patch on top of my patches for VFS switches. The
change was adding v9fs_statfs pointer to v9fs_super_ops_dotl
instead of v9fs_super_ops.

statfs - get file system statistics

size[4] Tstatfs tag[2] fid[4]
size[4] Rstatfs tag[2] type[4] bsize[4] blocks[8] bfree[8] bavail[8]
                files[8] ffree[8] fsid[8] namelen[4]

The statfs message is used to request file system information returned
by the statfs(2) system call, which is used by df(1) to report file
system and disk space usage.

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