Linux 4.18: Use ktime_get_coarse_real_ts64()

Newer kernels remove current_kernel_time64().  Use
ktime_get_coarse_real_ts64() in its place.

Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes: openzfs#8258

config/kernel-ktime_get_coarse_real_ts64.m4

@@ -0,0 +1,18 @@
+dnl #
+dnl # 4.18: ktime_get_coarse_real_ts64() added.  Use it in place of
+dnl # current_kernel_time64().
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_KTIME_GET_COARSE_REAL_TS64],
+	[AC_MSG_CHECKING([whether ktime_get_coarse_real_ts64() exists])
+	ZFS_LINUX_TRY_COMPILE([
+		#include <linux/mm.h>
+	], [
+		struct timespec64 ts;
+		ktime_get_coarse_real_ts64(&ts);
+	], [
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_KTIME_GET_COARSE_REAL_TS64, 1, [ktime_get_coarse_real_ts64() exists])
+	], [
+		AC_MSG_RESULT(no)
+	])
+])

config/kernel.m4

@@ -161,6 +161,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_ACL_HAS_REFCOUNT
 	ZFS_AC_KERNEL_USERNS_CAPABILITIES
 	ZFS_AC_KERNEL_IN_COMPAT_SYSCALL
+	ZFS_AC_KERNEL_KTIME_GET_COARSE_REAL_TS64
 
 	AS_IF([test "$LINUX_OBJ" != "$LINUX"], [
 		KERNEL_MAKE="$KERNEL_MAKE O=$LINUX_OBJ"

include/spl/sys/time.h

@@ -73,7 +73,13 @@ static inline void
 gethrestime(inode_timespec_t *ts)
 {
 #if defined(HAVE_INODE_TIMESPEC64_TIMES)
+
+#if defined(HAVE_KTIME_GET_COARSE_REAL_TS64)
+	ktime_get_coarse_real_ts64(ts);
+#else
 	*ts = current_kernel_time64();
+#endif /* HAVE_KTIME_GET_COARSE_REAL_TS64 */
+
 #else
 	*ts = current_kernel_time();
 #endif
@@ -83,7 +89,13 @@ static inline time_t
 gethrestime_sec(void)
 {
 #if defined(HAVE_INODE_TIMESPEC64_TIMES)
+#if defined(HAVE_KTIME_GET_COARSE_REAL_TS64)
+	inode_timespec_t ts;
+	ktime_get_coarse_real_ts64(&ts);
+#else
 	inode_timespec_t ts = current_kernel_time64();
+#endif  /* HAVE_KTIME_GET_COARSE_REAL_TS65 */
+
 #else
 	inode_timespec_t ts = current_kernel_time();
 #endif