jbd2: export jbd2_journal_[un]register_shrinker()

Export jbd2_journal_[un]register_shrinker() to fix this error when
ext4 is built as a module:

  ERROR: modpost: "jbd2_journal_unregister_shrinker" undefined!
  ERROR: modpost: "jbd2_journal_register_shrinker" undefined!

Fixes: 4ba3fcd ("jbd2,ext4: add a shrinker to release checkpointed buffers")
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20210630083638.140218-1-yi.zhang@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/jbd2/journal.c

@@ -2122,6 +2122,7 @@ int jbd2_journal_register_shrinker(journal_t *journal)
 
 	return 0;
 }
+EXPORT_SYMBOL(jbd2_journal_register_shrinker);
 
 /**
  * jbd2_journal_unregister_shrinker()
@@ -2134,6 +2135,7 @@ void jbd2_journal_unregister_shrinker(journal_t *journal)
 	percpu_counter_destroy(&journal->j_jh_shrink_count);
 	unregister_shrinker(&journal->j_shrinker);
 }
+EXPORT_SYMBOL(jbd2_journal_unregister_shrinker);
 
 /**
  * jbd2_journal_destroy() - Release a journal_t structure.