nfs: fix timstamp debug prints

Starting in v5.5, the timestamps are correctly passed down as
64-bit seconds with NFSv4 on 32-bit machines, but some debug
statements still truncate them to 'long'.

Fixes: e86d5a02 ("NFS: Convert struct nfs_fattr to use struct timespec64")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>