The existing folios macros assume page-aligned memory, but U-Boot uses
malloc'd buffers for simplicity.
Update the macros accordinging:
- offset_in_folio(): Calculate the offset from the folio's data pointer
- bh_offset(): Calculate the actual offset within the folio
- folio_set_bh(): Actually set b_folio and b_data
- kmap_local_folio(): Return a pointer to folio data + offset
Implement __filemap_get_folio(), folio_put() and folio_get() for
folio-lifecycle management.
Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>
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