Linux 6.7 compat: simplify current_time() check

6.7 changed the names of the time members in struct inode, so we can't
assign back to it because we don't know its name. In practice this
doesn't matter though - if we're missing current_time(), then we must be
on <4.9, and we know our fallback will need to return timespec.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://github.com/sponsors/robn
Closes openzfs#15681

config/kernel-current-time.m4

@@ -2,12 +2,15 @@ dnl #
 dnl # 4.9, current_time() added
 dnl # 4.18, return type changed from timespec to timespec64
 dnl #
+dnl # Note that we don't care about the return type in this check. If we have
+dnl # to implement a fallback, we'll know we're <4.9, which was timespec.
+dnl #
 AC_DEFUN([ZFS_AC_KERNEL_SRC_CURRENT_TIME], [
 	ZFS_LINUX_TEST_SRC([current_time], [
 		#include <linux/fs.h>
 	], [
 		struct inode ip __attribute__ ((unused));
-		ip.i_atime = current_time(&ip);
+		(void) current_time(&ip);
 	])
 ])