fs: nfs: Make nfs boot time y2038 safe

boot_time is represented as a struct timespec.
struct timespec and CURRENT_TIME are not y2038 safe.
Overall, the plan is to use timespec64 and ktime_t for
all internal kernel representation of timestamps.
CURRENT_TIME will also be removed.

boot_time is used to construct the nfs client boot verifier.

Use ktime_t to represent boot_time and ktime_get_real() for
the boot_time value.

Following Trond's request https://lkml.org/lkml/2016/6/9/22

 ,
use ktime_t instead of converting to struct timespec64.

Use higher and lower 32 bit parts of ktime_t for the boot
verifier.

Use the lower 32 bit part of ktime_t for the authsys_parms
stamp field.

Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Cc: Trond Myklebust <trond.myklebust@primarydata.com>
Cc: Anna Schumaker <anna.schumaker@netapp.com>
Cc: linux-nfs@vger.kernel.org
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>