Fix a wrong function name in backend.DrbdAttachNet

Commit cf8df3f3 "bdev: forward-port
ReAttachNet/DisconnectNet" forward-ported 1.2's bdev.DRBD8.ReAttachNet()
to 2.0 while renaming it to AttachNet(), but commit
6b93ec9d "Forward-port DrbdNetReconfig"
didn't rename all the calls to it and left one ReAttachNet call in
backend.py.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>