rdma: update default number of shared buffers.

When the decision was made to uncouple the number of shared buffers from
the queue depth and allow the user to decide for themselves, the default
was also significantly lowered, which caused some issues when trying
torun performance tests (See https://github.com/spdk/spdk/issues/699

).
While this is a user modifiable variable, it is still best to keep the
higher default value.

The original value was equivalent to max_queue_depth *
SPDK_NVMF_MAX_SGL_ENTRIES * 2 with the defaults for max_queue depth and
max_sgl_entries being 128 and 16 respectively. Hence 4096

fixes: 0b20f2e5

Change-Id: I809e97a10973093a2b485b85bca7160091166f70
Signed-off-by: Seth Howell <seth.howell@intel.com>
Reviewed-on: https://review.gerrithub.io/c/446525


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>