glusterfs: Use github instead of download.gluster.org to obtain source.

This is because the source tarball available on
  https://download.gluster.org/pub/gluster/glusterfs/3.10/3.10.1/glusterfs-3.10.1.tar.gz
has different contents than the v3.10.1 tag;
for example, it lacks the file `xlators/features/ganesha/src/Makefile.am`,
which the tag has.
This is because GluserFS's release process removes some unused files.

This made impossible to apply patches written by or for upstream, as those
are written against what's in upstream's git.

As a nice side effect, we no longer have to hardcode the "3.10" in the
`3.10/${version}` part of the URL.