9p: create a v9fs_context structure to hold parsed options

This patch creates a new v9fs_context structure which includes
v9fs_session_info as well as p9_client, p9_fd_opts, and p9_rdma_opts
to hold all parsed options. The new structure will be used in the next
commit to pass all parsed options to the appropriate transports.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>