SUNRPC: Refactor svc_recvfrom()

This function is not currently "generic" so remove the documenting
comment and rename it appropriately. Its internals are converted to
use bio_vecs for reading from the transport socket.

In existing typical sunrpc uses of bio_vecs, the bio_vec array is
allocated dynamically. Here, instead, an array of bio_vecs is added
to svc_rqst. The lifetime of this array can be greater than one call
to xpo_recvfrom():

- Multiple calls to xpo_recvfrom() might be needed to read an RPC
  message completely.

- At some later point, rq_arg.bvecs will point to this array and it
  will carry the received message into svc_process().

I also expect that a future optimization will remove either the
rq_vec or rq_pages array in favor of rq_bvec, thus conserving the
size of struct svc_rqst.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

include/linux/sunrpc/svc.h

@@ -254,6 +254,7 @@ struct svc_rqst {
 	struct page *		*rq_page_end;  /* one past the last page */
 
 	struct kvec		rq_vec[RPCSVC_MAXPAGES]; /* generally useful.. */
+	struct bio_vec		rq_bvec[RPCSVC_MAXPAGES];
 
 	__be32			rq_xid;		/* transmission id */
 	u32			rq_prog;	/* program number */