Fix TestSafeLink on ARM64

[Honny1]

This PR fixes the TestSafeLink test which fails on ARM64.

The failing test:

--- FAIL: TestSafeLink (0.00s)
    filesystem_linux_test.go:176: 
        	Error Trace:	/root/projects/containers/storage/pkg/chunked/filesystem_linux_test.go:176
        	Error:      	Not equal: 
        	            	expected: uint32(0x2)
        	            	actual  : uint64(0x2)
        	Test:       	TestSafeLink 

This is because the Stat_t structure has different data types, see arm version. The same problem is solved in the function getNlink().

[rhatdan]

/approve
/lgtm
Thanks @Honny1

[openshift-ci]

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This pull-request has been approved by: Honny1, rhatdan

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