ext4: clarify return semantics of ext4_load_tail_bh()

ext4_load_tail_bh() returns NULL for both holes and clean unwritten
buffers, but the conditions that lead to this are not obvious from the
code alone. Document this behavior to clarify the return value, so that
readers do not mistakenly assume that only holes result in a NULL
return.

Also update the inline comment following the ext4_get_block() call to
reflect this, and note that a lookup-only get_block (without
EXT4_GET_BLOCKS_CREATE) never sets BH_Mapped for clean unwritten
extents, which is why a clean unwritten bh falls through to the
"nothing to do" path.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20260714080044.4038124-6-yi.zhang@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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