mbcache: Add functions to delete entry if unused

hulk inclusion
category: bugfix
bugzilla: 186975, https://gitee.com/openeuler/kernel/issues/I5HT6F
CVE: NA

Reference: https://patchwork.ozlabs.org/project/linux-ext4/list/?series=309169



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Add function mb_cache_entry_delete_or_get() to delete mbcache entry if
it is unused and also add a function to wait for entry to become unused
- mb_cache_entry_wait_unused(). We do not share code between the two
deleting function as one of them will go away soon.

CC: stable@vger.kernel.org
Fixes: 82939d79 ("ext4: convert to mbcache2")
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>