fs: add a FMODE_ flag to indicate IOCB_HAS_METADATA availability

Currently the kernel will happily route io_uring requests with metadata
to file operations that don't support it.  Add a FMODE_ flag to guard
that.

Fixes: 4de2ce04 ("fs: introduce IOCB_HAS_METADATA for metadata")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/20250819082517.2038819-2-hch@lst.de


Signed-off-by: Christian Brauner <brauner@kernel.org>