Linux 4.18 compat: inode timespec -> timespec64

Commit torvalds/linux@95582b0 changes the inode i_atime, i_mtime,
and i_ctime members form timespec's to timespec64's to make them
2038 safe. As part of this change the current_time() function was
also updated to return the timespec64 type.

Resolve this issue by introducing a new inode_timespec_t type which
is defined to match the timespec type used by the inode. It should
be used when working with inode timestamps to ensure matching types.

The timestruc_t type under Illumos was used in a similar fashion but
was specified to always be a timespec_t. Rather than incorrectly
define this type all timespec_t types have been replaced by the new
inode_timespec_t type.

Finally, the kernel and user space 'sys/time.h' headers were aligned
with each other. They define as appropriate for the context several
constants as macros and include static inline implementation of
gethrestime(), gethrestime_sec(), and gethrtime().

Reviewed-by: Chunwei Chen <tuxoko@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7643