Move zfs_version_kernel to platform code

Linux uses sysfs to determine the module version, FreeBSD uses a
different method.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #9978

lib/libzfs/os/linux/libzfs_util_os.c

@@ -41,6 +41,7 @@
 #include "libzfs_impl.h"
 #include "zfs_prop.h"
 #include <libzutil.h>
+#include <sys/zfs_sysfs.h>
 
 #define	ZDIFF_SHARESDIR		"/.zfs/shares/"
 
@@ -182,3 +183,33 @@ find_shares_object(differ_info_t *di)
 	di->shares = (uint64_t)sb.st_ino;
 	return (0);
 }
+
+/*
+ * Fill given version buffer with zfs kernel version read from ZFS_SYSFS_DIR
+ * Returns 0 on success, and -1 on error (with errno set)
+ */
+int
+zfs_version_kernel(char *version, int len)
+{
+	int _errno;
+	int fd;
+	int rlen;
+
+	if ((fd = open(ZFS_SYSFS_DIR "/version", O_RDONLY)) == -1)
+		return (-1);
+
+	if ((rlen = read(fd, version, len)) == -1) {
+		version[0] = '\0';
+		_errno = errno;
+		(void) close(fd);
+		errno = _errno;
+		return (-1);
+	}
+
+	version[rlen-1] = '\0';  /* discard '\n' */
+
+	if (close(fd) == -1)
+		return (-1);
+
+	return (0);
+}