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define this type all timespec_t types have been replaced by the new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
Backported-by: Richard Yao <ryao@gentoo.org>

config/kernel-current-time.m4

@@ -1,15 +1,14 @@
 dnl #
 dnl # 4.9, current_time() added
+dnl # 4.18, return type changed from timespec to timespec64
 dnl #
 AC_DEFUN([ZFS_AC_KERNEL_CURRENT_TIME],
 	[AC_MSG_CHECKING([whether current_time() exists])
 	ZFS_LINUX_TRY_COMPILE_SYMBOL([
 		#include <linux/fs.h>
 	], [
-		struct inode ip;
-		struct timespec now __attribute__ ((unused));
-
-		now = current_time(&ip);
+		struct inode ip __attribute__ ((unused));
+		ip.i_atime = current_time(&ip);
 	], [current_time], [fs/inode.c], [
 		AC_MSG_RESULT(yes)
 		AC_DEFINE(HAVE_CURRENT_TIME, 1, [current_time() exists])