jbd2: export jbd2_journal_[un]register_shrinker()

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: 4ba3fcdde7e3 ("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>