gmi.c:
Get rid of magic number 1500 and replace with PACKET_SIZE_MAX.
Also fix a glaring memory overwrite bug in gmi_init which would
init 256k of memory instead of PACKET_SIZE_MAX memory.

BKrev: 413e7bc48wgkqG1E4wSbG9KQkBJMkg


git-svn-id: http://svn.fedorahosted.org/svn/corosync/trunk@241 fd59a12c-fef9-0310-b244-a6a79926bd2f