backtrace: rework for signal safety

Mostly, try a lot harder to not allocate anything.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16181

lib/libspl/assert.c

@@ -103,7 +103,7 @@ libspl_assertf(const char *file, const char *func, int line,
 	    getpid(), libspl_getprogname(),
 	    libspl_gettid(), tname);
 
-	libspl_backtrace();
+	libspl_backtrace(STDERR_FILENO);
 
 #if !__has_feature(attribute_analyzer_noreturn) && !defined(__COVERITY__)
 	if (libspl_assert_ok) {