fs: nfs: get rid of memcpys for inode times

Subsequent patches in the series convert inode timestamps
to use struct timespec64 instead of struct timespec as
part of solving the y2038 problem.
This will lead to type mismatch for memcpys.
Use regular assignments instead.

Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
Cc: trond.myklebust@primarydata.com