jbd2: point the shadow buffer at the frozen data directly

When a metadata buffer has to be copied out before it can be journalled,
jbd2_journal_write_metadata_buffer() writes jh->b_frozen_data rather than
the page cache copy.  b_frozen_data is kmalloc()ed, so folio_set_bh() makes
the shadow buffer point at a slab folio.

That is not something the buffer_head layer can reason about.  A slab folio
overloads ->mapping, so a shadow buffer looks like it belongs to an
address_space when it does not.  buffer_set_crypto_ctx() already has to
work around this, and it is the reason mark_buffer_write_io_error() cannot
be called on a shadow buffer today.

Point the shadow buffer at the frozen data itself instead: leave b_folio
NULL, which it already is out of alloc_buffer_head(), and set b_data.  The
previous patch taught fs/buffer.c to submit such a buffer.  folio_set_bh()
is now needed on only one path - the one that journals the page cache copy
directly - so it moves there, and new_folio, new_offset and the flag that
used to pick between them all go away.

The two commit-path checksum helpers reach the shadow buffer's contents
through a new kmap_local_bh()/kunmap_local_bh() pair, which handle a buffer
with or without a folio.  Memory outside the page cache is always mapped,
so for those there is nothing to map or unmap.  Mapping it anyway would be
worse than pointless: with CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP,
kmap_local_page() hands back a one page mapping even for such memory, which
is not enough for a buffer bigger than a page.

Tested with ext4 mounted data=journal,journal_checksum on a metadata_csum
filesystem, writing files whose every block begins with the JBD2 magic so
that escaping forces the copy-out, then crashing with sysrq-b without
unmounting and replaying the journal on the next mount.  Recovery
completed, the file contents matched, e2fsck -fn was clean, and an
instrumented build confirmed the b_folio == NULL path was taken.

Suggested-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Weidong Zhu <weizhu@fiu.edu>
Signed-off-by: Chao Shi <coshi036@gmail.com>
Link: https://patch.msgid.link/6140cd23beb88e99f40eaeff4044a16213f6caab.1785951556.git.coshi036@gmail.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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