hostfs: pass 64-bit timestamps to/from user space

The use of 'struct timespec' is deprecated in the kernel, so we
want to avoid the conversions from/to the proper timespec64
structure.

On the user space side, we have a 'struct timespec' that is defined
by the C library and that will be incompatible with the kernel's
view on 32-bit architectures once they move to a 64-bit time_t,
breaking the shared binary layout of hostfs_iattr and hostfs_stat.

This changes the two structures to use a new hostfs_timespec structure
with fixed 64-bit seconds/nanoseconds for passing the timestamps
between hostfs_kern.c and hostfs_user.c. With a new enough user
space side, this will allow timestamps beyond year 2038.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>