nfsd: print 64-bit timestamps in client_info_show

The nii_time field gets truncated to 'time_t' on 32-bit architectures
before printing.

Remove the use of 'struct timespec' to product the correct output
beyond 2038.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>