ext4: introduce new metadata buffer read helpers

The previous patch add clear_buffer_verified() before we read metadata
block from disk again, but it's rather easy to miss clearing of this bit
because currently we read metadata buffer through different open codes
(e.g. ll_rw_block(), bh_submit_read() and invoke submit_bh() directly).
So, it's time to add common helpers to unify in all the places reading
metadata buffers instead. This patch add 3 helpers:

 - ext4_read_bh_nowait(): async read metadata buffer if it's actually
   not uptodate, clear buffer_verified bit before read from disk.
 - ext4_read_bh(): sync version of read metadata buffer, it will wait
   until the read operation return and check the return status.
 - ext4_read_bh_lock(): try to lock the buffer before read buffer, it
   will skip reading if the buffer is already locked.

After this patch, we need to use these helpers in all the places reading
metadata buffer instead of different open codes.

Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Suggested-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20200924073337.861472-3-yi.zhang@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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