truncate,shmem: Handle truncates that split large folios

truncate,shmem: Handle truncates that split large folios

Handle folio splitting in the parts of the truncation functions which
already handle partial pages.  Factor all that code out into a new
function called truncate_inode_partial_folio().

Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Reviewed-by: William Kucharski <[email protected]>