scripts: switch from go fmt to gofmt

Neither "go fmt" nor "gofmt" return a status code when the format check
fails. We can only rely on their stdout. "go fmt" invokes "gofmt -w -l"
on every file in the target package and pulls in dependencies. These
dependencies are not verified, but they pollute stdout and increase
format check time.

Therefore, we switch to "gofmt", which only checks files within the
target package.

It turns out that not all files from the target package were checked.
Compare "go fmt -n ." to "go fmt -n ./...". Switching to "gofmt ."
addresses this issue, and we've added fixes to client.go accordingly.

Finally, instead of fixing the formatting, we print out the diff by
switching from "-w" to "-d".

Change-Id: Ifb1c339bdd1703c7d9e1b0cfa0b0ad0df7cff683
Signed-off-by: Emilia Haligowska <emilia.haligowska@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/22832


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Konrad Sztyber <konrad.sztyber@intel.com>
Community-CI: Mellanox Build Bot