ceph.spec.in: increase the mem_per_job to 3GiB

ceph.spec.in: increase the mem_per_job to 3GiB

to lower the number of jobs, we are experiencing build failures on
a builder with 48c96t, 193 free mem. the failures were caused by
OOM killer which kills the c++ compiler

[498376.128969] oom-kill:constraint=CONSTRAINT_NONE,nodemask=(null),cpuset=/,mems_allowed=0,global_oom,task_memcg=/system.slice/jenkins.service,task=cc1plus,pid=1387895,uid=1110
[498376.145288] Out of memory: Killed process 1387895 (cc1plus) total-vm:3323312kB, anon-rss:3164568kB, file-rss:0kB, shmem-rss:0kB, UID:1110
[498376.315185] oom_reaper: reaped process 1387895 (cc1plus), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB
[498377.882072] cc1plus invoked oom-killer: gfp_mask=0x6200ca(GFP_HIGHUSER_MOVABLE), order=0, oom_score_adj=0

before this change, we use the total memory to calculate the number
of jobs, and assume that each job takes at most 2.5GiB mem. in the
case above, the # of job is 96.

after this change, we use the free memory, and increse the mem per job
to 3.0GiB. in the case above, the # of job would be 85.

Signed-off-by: Kefu Chai <kchai@redhat.com>