btrfs: separate definition of assertion failure handlers

There's a report where objtool detects unreachable instructions, eg.:

fs/btrfs/ctree.o: warning: objtool: btrfs_search_slot()+0x2d4: unreachable instruction

This seems to be a false positive due to compiler version. The cause is
in the ASSERT macro implementation that does the conditional check as
IS_DEFINED(CONFIG_BTRFS_ASSERT) and not an #ifdef.

To avoid that, use the ifdefs directly.

There are still 2 reports that aren't fixed:

fs/btrfs/extent_io.o: warning: objtool: __set_extent_bit()+0x71f: unreachable instruction
fs/btrfs/relocation.o: warning: objtool: find_data_references()+0x4e0: unreachable instruction

Co-developed-by: Josh Poimboeuf <[e-mail]>
Signed-off-by: Josh Poimboeuf <[e-mail]>
Reported-by: Randy Dunlap <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/ctree.h

···
 	rcu_read_unlock();					\
 } while (0)
 
-__cold
-static inline void assfail(const char *expr, const char *file, int line)
+#ifdef CONFIG_BTRFS_ASSERT
+__cold __noreturn
+static inline void assertfail(const char *expr, const char *file, int line)
 {
-	if (IS_ENABLED(CONFIG_BTRFS_ASSERT)) {
-		pr_err("assertion failed: %s, in %s:%d\n", expr, file, line);
-		BUG();
-	}
+	pr_err("assertion failed: %s, in %s:%d\n", expr, file, line);
+	BUG();
 }
 
-#define ASSERT(expr)	\
-	(likely(expr) ? (void)0 : assfail(#expr, __FILE__, __LINE__))
+#define ASSERT(expr)						\
+	(likely(expr) ? (void)0 : assertfail(#expr, __FILE__, __LINE__))
+
+#else
+static inline void assertfail(const char *expr, const char* file, int line) { }
+#define ASSERT(expr)	(void)(expr)
+#endif
 
 /*
  * Use that for functions that are conditionally exported for sanity tests but