nas gateway: fix regression in global bucket policy initialization (#6243)
nas gateway: fix regression in global bucket policy initialization (#6243)

Fixes #6238
globalPolicySys used to be initialized in fs/xl layer. The referenced
commit moved this logic to server/gateway initialization,but a check
to avoid double initialization prevented globalPolicySys to be loaded
from disk for NAS.

fixes regression from commit be1700f5950b5630ef3bfd81a0785f350559ebfe
aead/minio,aead/minio,fwessels/minio,harshavardhana/minio,aead/minio,krisis/minio,aead/minio,aead/minio,jimjag/minio,harshavardhana/minio,minio/minio,NitishT/minio,fwessels/minio,donatello/minio,krishnasrinivas/minio,fwessels/minio,balamurugana/minio,minio/minio,NitishT/minio,harshavardhana/minio,NitishT/minio,jimjag/minio,jimjag/minio,harshavardhana/minio,vadmeste/minio,vadmeste/minio,balamurugana/minio,aead/minio,krishnasrinivas/minio,harshavardhana/minio,NitishT/minio,krisis/minio,harshavardhana/minio,harshavardhana/minio,krishnasrinivas/minio,fwessels/minio,jimjag/minio,harshavardhana/minio,aead/minio,krishnasrinivas/minio,donatello/minio,aead/minio