backtrace: fix off-by-one on string output

sizeof("foo") includes the trailing null byte, so all the output had
nulls through it. Most terminals quietly ignore it, but it makes some
tools misdetect file types and other annoyances.

Easy fix: subtract 1.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Alexander Motin <[email protected]>
Signed-off-by: Rob Norris <[email protected]>
Closes openzfs#16862

lib/libspl/backtrace.c

@@ -38,7 +38,7 @@
  */
 #define	spl_bt_write_n(fd, s, n) \
 	do { ssize_t r __maybe_unused = write(fd, s, n); } while (0)
-#define	spl_bt_write(fd, s)		spl_bt_write_n(fd, s, sizeof (s))
+#define	spl_bt_write(fd, s)		spl_bt_write_n(fd, s, sizeof (s)-1)
 
 #if defined(HAVE_LIBUNWIND)
 #define	UNW_LOCAL_ONLY