Add the correct offset to folio_zero_tail().

Fixes: d86f2de0 ("romfs: Convert romfs_read_folio() to use a folio")
Reported-by: Greg Ungerer <gregungerer@westnet.com.au>
Link: https://lore.kernel.org/r/Zr0GTnPHfeA0P8nb@casper.infradead.orgSigned-off-by: Christian Brauner <brauner@kernel.org>