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define this type all timespec_t types have been replaced by the new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

config/kernel-inode-times.m4

@@ -0,0 +1,25 @@
+dnl #
+dnl # 4.18 API change
+dnl # i_atime, i_mtime, and i_ctime changed from timespec to timespec64.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_INODE_TIMES], [
+	AC_MSG_CHECKING([whether inode->i_*time's are timespec64])
+	tmp_flags="$EXTRA_KCFLAGS"
+	EXTRA_KCFLAGS="-Werror"
+	ZFS_LINUX_TRY_COMPILE([
+		#include <linux/fs.h>
+	],[
+		struct inode ip;
+		struct timespec ts;
+
+		memset(&ip, 0, sizeof(ip));
+		ts = ip.i_mtime;
+	],[
+		AC_MSG_RESULT(no)
+	],[
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_INODE_TIMESPEC64_TIMES, 1,
+		    [inode->i_*time's are timespec64])
+	])
+	EXTRA_KCFLAGS="$tmp_flags"
+])