xfs: validate logged dquot buffers during recovery

A dquot buffer is only ever logged by xfs_qm_init_dquot_blk(), which initialises
and logs the whole buffer in a single region. So a logged dquot buffer has
exactly two regions, the format header and one data region covering the entire
buffer, and that region holds a full chunk of valid dquots.

Validate this in the buffer item validate_item callback. A buffer carrying a
dquot type flag must have that shape, and every dquot in the logged chunk is
verified so a corrupt dquot is never replayed to disk. The dquot content check
in the replay path only looked at the first dquot and relied on the replay loop
for the region bounds, so it is removed and the replay loop reduces to copying
the logged regions into the buffer.

To share the per-dquot verification, xfs_dquot_verify_chunk() is factored out of
xfs_dquot_buf_verify(). It walks a chunk of dquots and, on failure, updates its
dquot pointer to the offending dquot so the caller can report it. The buffer
read verifier is otherwise unchanged.

Assisted-by: LLM
Signed-off-by: Dave Chinner <dgc@kernel.org>