freevxfs: Convert vxfs_immed_read_folio() to use a folio

Reorganise the file to remove the forward declaration.
Use folios throughout vxfs_immed_read_folio().
Use memcpy_to_page() instead of an open-coded kmap()/kunmap().
Remove flush_dcache_page() as this is embedded in memcpy_to_page().
Use folio_pos() instead of opencoding it.
Handle multi-page folios.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>