Make "-fno-inline" compile option more accessible

When functions are inlined, it can make the system much more difficult
to instrument using tools such as ftrace, BPF, crash, etc. Thus, to aid
development and increase the system's observability, when the
"--enable-debuginfo" flag is specified, the "-fno-inline" compilation
option will be used for both userspace and kernel modules.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Prakash Surya <prakash.surya@delphix.com>
Closes #6605

config/zfs-build.m4

@@ -45,10 +45,11 @@ AC_DEFUN([ZFS_AC_DEBUG], [
 
 AC_DEFUN([ZFS_AC_DEBUGINFO_KERNEL], [
 	KERNELMAKE_PARAMS="$KERNELMAKE_PARAMS CONFIG_DEBUG_INFO=y"
+	KERNELCPPFLAGS="${KERNELCPPFLAGS} -fno-inline"
 ])
 
 AC_DEFUN([ZFS_AC_DEBUGINFO_USER], [
-	DEBUG_CFLAGS="$DEBUG_CFLAGS -g"
+	DEBUG_CFLAGS="${DEBUG_CFLAGS} -g -fno-inline"
 ])
 
 AC_DEFUN([ZFS_AC_DEBUGINFO], [