SUNRPC: skb_read_bits is the same as xs_tcp_copy_data

Clean-up: eliminate xs_tcp_copy_data -- it's exactly the same logic as the
common routine skb_read_bits.  The UDP and TCP socket read code now share
the same routine for copying data into an xdr_buf.

Now that skb_read_bits() is exported, rename it to avoid confusing it with
a generic skb_* function.  As these functions are XDR-specific, they should
not have names that suggest they are of generic use.  Also rename
skb_read_and_csum_bits() to be consistent.

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