ec: Fix incorrect value of EC_MAX_NODES

EC_MAX_NODES was incorrectly calculated. Now the value if computed
as the minimum between the theoretical maximum and the limit imposed
by the Galois Field.

This is a backport of http://review.gluster.org/9193/

Change-Id: I75a8345147f344f051923d66be2c10d405370c7b
BUG: 1170959
Signed-off-by: Xavier Hernandez <xhernandez@datalab.es>
Reviewed-on: http://review.gluster.org/9245
Reviewed-by: Dan Lambright <dlambrig@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Raghavendra Bhat <raghavendra@redhat.com>
Tested-by: Raghavendra Bhat <raghavendra@redhat.com>