files.c: fix printf format warnings

files.c: In function ‘collect_fd’:
files.c:111:2: error: format ‘%d’ expects type ‘int’, but argument 3 has type ‘u64’
files.c: In function ‘open_fd’:
files.c:348:3: error: format ‘%d’ expects type ‘int’, but argument 2 has type ‘u64’
files.c: In function ‘receive_fd’:
files.c:425:5: error: format ‘%d’ expects type ‘int’, but argument 4 has type ‘u64’
files.c:425:5: error: format ‘%d’ expects type ‘int’, but argument 5 has type ‘u64’

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>