Move range_tree, btree, highbit64 to common code

Break out the range_tree, btree, and highbit64/lowbit64 code from kernel
space into shared kernel and userspace code.  This is needed for the
updated `zpool status -vv` error byte range reporting that will be
coming in a future commit.  That commit needs the range_tree code in
kernel and userspace.

Reviewed-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #18133

lib/libspl/include/sys/sysmacros.h

@@ -30,6 +30,7 @@
 #define	_LIBSPL_SYS_SYSMACROS_H
 
 #include <stdint.h>
+#include <limits.h>
 
 #ifdef __linux__
 /*
@@ -120,7 +121,31 @@
 #define	CPU_SEQID	((uintptr_t)pthread_self() & (max_ncpus - 1))
 #define	CPU_SEQID_UNSTABLE	CPU_SEQID
 
-extern int lowbit64(uint64_t i);
-extern int highbit64(uint64_t i);
+/*
+ * Find highest one bit set.
+ * Returns bit number + 1 of highest bit that is set, otherwise returns 0.
+ */
+static inline int
+highbit64(uint64_t i)
+{
+	if (i == 0)
+		return (0);
+
+	return (CHAR_BIT * sizeof (uint64_t) - __builtin_clzll(i));
+}
+
+/*
+ * Find lowest one bit set.
+ * Returns bit number + 1 of lowest bit that is set, otherwise returns 0.
+ * The __builtin_ffsll() function is supported by both GCC and Clang.
+ */
+static inline int
+lowbit64(uint64_t i)
+{
+	if (i == 0)
+		return (0);
+
+	return (__builtin_ffsll(i));
+}
 
 #endif /* _SYS_SYSMACROS_H */