iomap: introduce IOMAP_F_FSVERITY and teach writeback to handle fsverity

iomap: introduce IOMAP_F_FSVERITY and teach writeback to handle fsverity

This flag indicates that I/O is for fsverity metadata.

In the write path skip i_size check and i_size updates as metadata is
past EOF. In writeback don't update i_size and continue writeback if
even folio is beyond EOF. In read path don't zero fsverity folios, again
they are past EOF.

The iomap_block_needs_zeroing() is also called from write path. For
folios of larger order we don't want to zero out pages in the folio as
these could contain other merkle tree blocks. For fsverity, filesystem
will request to read PAGE_SIZE memory regions. For data folios, iomap
will zero the rest of the folio for anything which is beyond EOF. We
don't want this for fsverity folios.

Christian Brauner <brauner@kernel.org> says:
Changed IOMAP_F_FSVERITY from (1U << 10) to (1U << 11) to avoid colliding
with IOMAP_F_ZERO_TAIL, which already uses (1U << 10).

Signed-off-by: Andrey Albershteyn <aalbersh@kernel.org>
Link: https://patch.msgid.link/20260520123722.405752-8-aalbersh@kernel.org
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>