fuse: refactor writeback to use iomap_writepage_ctx inode

struct iomap_writepage_ctx includes a pointer to the file inode. In
writeback, use that instead of also passing the inode into
fuse_fill_wb_data.

No functional changes.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Link: https://lore.kernel.org/20250715202122.2282532-6-joannelkoong@gmail.com


Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>