arc_os: split userspace and Linux kernel code

The Linux arc_os.c carries userspace and kernel code, with very little
overlap between the two. This lifts the userspace parts out into a
separate arc_os.c for libzpool and removes it from the Linux side.

Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #16492

lib/libzpool/Makefile.am

@@ -12,6 +12,7 @@ CPPCHECKTARGETS += libzpool.la
 
 dist_libzpool_la_SOURCES = \
 	%D%/abd_os.c \
+	%D%/arc_os.c \
 	%D%/kernel.c \
 	%D%/taskq.c \
 	%D%/util.c
@@ -42,7 +43,6 @@ nodist_libzpool_la_SOURCES = \
 	module/lua/lvm.c \
 	module/lua/lzio.c \
 	\
-	module/os/linux/zfs/arc_os.c \
 	module/os/linux/zfs/trace.c \
 	module/os/linux/zfs/vdev_file.c \
 	module/os/linux/zfs/vdev_label_os.c \