librdmacm: returns errors from the library consistently

librdmacm: returns errors from the library consistently

Remove the return of -errno and always return codes via errno.
As documented in librdmacm, these libraries are already documented
to return -1 to indicate the code is in errno.

Update rping to show correct error reporting methodology.

Also fix errant return of 0 if the read/write syscalls return 0.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Sean Hefty <sean.hefty@intel.com>