net/9p: correct some comment errors in 9p file system code

There are follow comment errors:
1 The function name is wrong in p9_release_pages() comment.
2 The function name and variable name is wrong in p9_poll_workfn() comment.
3 There is no variable dm_mr and lkey in struct p9_trans_rdma.
4 The function name is wrong in rdma_create_trans() comment.
5 There is no variable initialized in struct virtio_chan.
6 The variable name is wrong in p9_virtio_zc_request() comment.

Signed-off-by: Sun Lianwen <sunlw.fnst@cn.fujitsu.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>