Fix clang 13 compilation errors

```
os/linux/zfs/zvol_os.c:1111:3: error: ignoring return value of function
  declared with 'warn_unused_result' attribute [-Werror,-Wunused-result]
                add_disk(zv->zv_zso->zvo_disk);
                ^~~~~~~~ ~~~~~~~~~~~~~~~~~~~~

zpl_xattr.c:1579:1: warning: no previous prototype for function
  'zpl_posix_acl_release_impl' [-Wmissing-prototypes]
```

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Closes openzfs#13551
(cherry picked from commit 9884319)

config/kernel-add-disk.m4

@@ -7,8 +7,7 @@ AC_DEFUN([ZFS_AC_KERNEL_SRC_ADD_DISK], [
 		#include <linux/blkdev.h>
 	], [
 		struct gendisk *disk = NULL;
-		int err = add_disk(disk);
-		err = err;
+		int error __attribute__ ((unused)) = add_disk(disk);
 	])
 ])
 

module/os/linux/zfs/zpl_xattr.c

@@ -1414,7 +1414,9 @@ zpl_xattr_handler(const char *name)
 	return (NULL);
 }
 
-#if !defined(HAVE_POSIX_ACL_RELEASE) || defined(HAVE_POSIX_ACL_RELEASE_GPL_ONLY)
+#if defined(CONFIG_FS_POSIX_ACL) && \
+	(!defined(HAVE_POSIX_ACL_RELEASE) || \
+		defined(HAVE_POSIX_ACL_RELEASE_GPL_ONLY))
 struct acl_rel_struct {
 	struct acl_rel_struct *next;
 	struct posix_acl *acl;