pkg/fs: optimize GetObject syscalls for common case
pkg/fs: optimize GetObject syscalls for common case

In the common case, GetObject is called on a bucket that exists and an
object that exists and is not a directory. It should be optimized for
this case, thus error-related syscalls are pushed back until they are
necessary.

This should not impact performance negatively in the uncommon case, and
instead drops two otherwise unnecessary os.Stat's in the common case.

The race conditions around a proper error being returned were present
beforehand.

It also renames 'err' to 'e'.

jimjag/minio,krisis/minio,krishnasrinivas/minio,rajivm/minio,donatello/minio,balamurugana/minio,aead/minio,dvstate/minio,krishnasrinivas/minio,hackintoshrao/minio,mf-00/newgo,rajivm/minio,harshavardhana/minio,harshavardhana/minio,luomeiqin/minio,aead/minio,rushenn/minio,NitishT/minio,dutchcoders/minio,donatello/minio,jimjag/minio,brendanashworth/minio,NitishT/minio,vadmeste/minio,abperiasamy/minio,krishnasrinivas/minio,krishnasrinivas/minio,koolhead17/minio,koolhead17/minio,abperiasamy/minio,NitishT/minio,jimjag/minio,brendanashworth/minio,vadmeste/minio,rushenn/minio,harshavardhana/minio,brendanashworth/minio,rushenn/minio,minio/minio,dutchcoders/minio,dvstate/minio,fwessels/minio,alexellis/minio,luomeiqin/minio,abperiasamy/minio,mkash32/minio,dutchcoders/minio,dutchcoders/minio,fwessels/minio,harshavardhana/minio,krisis/minio,aead/minio,aead/minio,hackintoshrao/minio,harshavardhana/minio,minio/minio,jimjag/minio,fwessels/minio,fwessels/minio,mf-00/newgo,aead/minio,dvstate/minio,hackintoshrao/minio,krishnasrinivas/minio-daily,aead/minio,abperiasamy/minio,aead/minio,mkash32/minio,harshavardhana/minio,NitishT/minio,krishnasrinivas/minio-daily,aead/minio,balamurugana/minio,rushenn/minio,luomeiqin/minio,harshavardhana/minio,hackintoshrao/minio,alexellis/minio,harshavardhana/minio,dvstate/minio,brendanashworth/minio,luomeiqin/minio