Optimize the _link_file function to not call os.stat redundantly.
    This will eliminate 2 calls to os.stat per one invocation of the _link_file function.
    Assuming during the compose build 50000 files are linked, this change will eliminate 100000 redundant calls to os.stat.
    
    Jira: RHELCMP-797
    Signed-off-by: Bohdan Khomutskyi <bkhomuts@redhat.com>