block/gluster: deprecate rdma support

block/gluster: deprecate rdma support

gluster volfile server fetch happens through unix and/or tcp, it doesn't
support volfile fetch over rdma. The rdma code may actually mislead,
so to make sure things do not break, for now we fallback to tcp when requested
for rdma, with a warning.

If you are wondering how this worked all these days, its the gluster libgfapi
code which handles anything other than unix transport as socket/tcp, sad but
true.

Also gluster doesn't support ipv6 addresses, removing the ipv6 related
comments/docs section

[Jeff: Minor grammatical fixes in comments and commit message, per
review comments]

Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-id: 1468947453-5433-4-git-send-email-prasanna.kalever@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>