fix maxmemory config warning
fix maxmemory config warning

the warning condition was if usage > limit (saying it'll cause eviction
or oom), but in fact the eviction and oom depends on used minus slave
buffers.

other than fixing the condition, i add info about the current usage and
limit, which may be useful when looking at the log.

PKRoma/redis,ofirluzon/redis,soloestoy/redis,oranagra/redis,PKRoma/redis,oranagra/redis,PKRoma/redis,charsyam/redis,charsyam/redis,yossigo/redis,neomantra/redis,soloestoy/redis,yossigo/redis,soloestoy/redis,antirez/redis,charsyam/redis,yossigo/redis,oranagra/redis,oranagra/redis,neomantra/redis,ofirluzon/redis,neomantra/redis,oranagra/redis,ofirluzon/redis,PKRoma/redis,charsyam/redis,yossigo/redis,antirez/redis,ctripcorp/redis,yossigo/redis,PKRoma/redis,ctripcorp/redis,antirez/redis,ctripcorp/redis,soloestoy/redis,soloestoy/redis,ofirluzon/redis,neomantra/redis,ofirluzon/redis,charsyam/redis,antirez/redis,neomantra/redis,ctripcorp/redis