truncate,shmem: Add truncate_inode_folio()

Convert all callers of truncate_inode_page() to call
truncate_inode_folio() instead, and move the declaration to mm/internal.h.
Move the assertion that the caller is not passing in a tail page to
generic_error_remove_page().  We can't entirely remove the struct page
from the callers yet because the page pointer in the pvec might be a
shadow/dax/swap entry instead of actually a page.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>