iomap: rework REQ_FUA selection

iomap: rework REQ_FUA selection

The way how iomap_dio_can_use_fua and the caller is structured is
a bit confusing, as the main guarding condition is hidden in the
helper, and the secondary conditions are split between caller and
callee.

Refactor the code, so that iomap_dio_bio_iter itself tracks if a write
might need metadata updates based on the iomap type and flags, and
then have a condition based on that to use the FUA flag.

Note that this also moves the REQ_OP_WRITE assignment to the end of
the branch to improve readability a bit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20251113170633.1453259-4-hch@lst.de
Signed-off-by: Christian Brauner <brauner@kernel.org>