Fix gometalint errors

[manishtomar]

Fix lint errors that occurred after recent change in gometalinter alecthomas/gometalinter#579

[manishtomar]

The only lint error is

$ make check
+ check
gometalinter --config .gometalinter.json ./...
registry/storage/garbagecollect.go:107:4:warning: when ok is true, err can't be nil (S1020) (staticcheck)
make: *** [check] Error 1

which I don't understand why since err is not checked after the type assertion: https://github.com/docker/distribution/blob/da8db4666b08431e701b044c9077648533d97b7e/registry/storage/garbagecollect.go#L107

[dmcgowan]

Try removing the err != nil check since the type check already covers that case

[codecov]

Codecov Report

Merging #2840 into master will increase coverage by 0.3%.
The diff coverage is 73.68%.

@@            Coverage Diff            @@
##           master    #2840     +/-   ##
=========================================
+ Coverage   60.15%   60.45%   +0.3%     
=========================================
  Files         103      102      -1     
  Lines        8038     8001     -37     
=========================================
+ Hits         4835     4837      +2     
+ Misses       2560     2517     -43     
- Partials      643      647      +4

Flag	Coverage Δ
#linux	60.45% <73.68%> (+0.3%)	⬆️

Impacted Files	Coverage Δ
registry/storage/blobstore.go	59.78% <ø> (+3.08%)	⬆️
registry/storage/tagstore.go	78.49% <ø> (+8.3%)	⬆️
registry/api/v2/urls.go	76.52% <ø> (+3.18%)	⬆️
registry/storage/schema2manifesthandler.go	65.57% <ø> (ø)	⬆️
registry/proxy/proxyblobstore.go	55.45% <ø> (ø)	⬆️
notifications/bridge.go	54.31% <ø> (+2.24%)	⬆️
notifications/sinks.go	95% <ø> (ø)	⬆️
registry/handlers/blobupload.go	44.72% <0%> (ø)	⬆️
manifest/ocischema/builder.go	66.66% <0%> (ø)	⬆️
registry/handlers/hooks.go	0% <0%> (ø)	⬆️
... and 21 more

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[dmcgowan]

LGTM