xprtrdma: Take struct ib_device_attr off the stack

Device attributes are large, and are used in more than one place.
Stash a copy in dynamically allocated memory.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>