xprtrdma: Modernize ops->connect

Adapt and apply changes that were made to the TCP socket connect
code. See the following commits for details on the purpose of
these changes:

Commit 7196dbb0 ("SUNRPC: Allow changing of the TCP timeout parameters on the fly")
Commit 3851f1cd ("SUNRPC: Limit the reconnect backoff timer to the max RPC message timeout")
Commit 02910177 ("SUNRPC: Fix reconnection timeouts")

Some common transport code is moved to xprt.c to satisfy the code
duplication police.

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