Fix go vet warnings

[magik6k]

Before:

$ go vet ./...                
core/builder.go:176: github.com/ipfs/go-ipfs/thirdparty/verifbs.VerifBS composite literal uses unkeyed fields
core/builder.go:205: github.com/ipfs/go-ipfs/thirdparty/verifbs.VerifBSGC composite literal uses unkeyed fields
exit status 1
filestore/fsrefstore.go:80: possible formatting directive in Error call
exit status 1
repo/fsrepo/fsrepo.go:622: self-assignment of value to value
exit status 1
test/dependencies/go-timeout/main.go:24: the cancel function returned by context.WithTimeout should be called, not discarded, to avoid a context leak
exit status 1
thirdparty/notifier/notifier.go:48: return copies lock value: notifier.Notifier contains sync.RWMutex
exit status 1