LU-7578 gnilnd: Revert max_immediate setting

LU-7578 gnilnd: Revert max_immediate setting

max_immediate was changed based on performance testing for
5.2UP04 and 6.0, this caused the eager_recv path to always use vmalloc
when allocating space for new eager messages. The vmalloc path is very
slow especially when constantly freeing at the same time across all
CPU's

This change will also cause more messages to be governed by the
service nodes rdma engine.

Modifications
max_immediate default is now 2048.
max_immediate is now read only.
eager_credits is now writeable at run time.

Signed-off-by: James Shimek <jshimek@cray.com>
Signed-off-by: Chris Horn <hornc@cray.com>
Change-Id: I2754d28b1f05a7aeaaeac7fc5f41f1f36568d79c
Reviewed-on: http://review.whamcloud.com/17667
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>