LU-10855 ptlrpc: assign specific values to MGS opcodes

Assign specific values to all of the MGS opcodes in enum mgs_cmd
so that these values do not change if a new items is added or one
is removed in the future.  These opcodes are part of the wire
protocol and need to remain constant.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I8132ca01916cd657933d0c8864e4e78f8b3ebbe5
Reviewed-on: https://review.whamcloud.com/32653
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>