iomap: support write completions from interrupt context

iomap: support write completions from interrupt context

Completions for pure overwrites don't need to be deferred to a workqueue
as there is no work to be done, or at least no work that needs a user
context.  Set the IOMAP_DIO_INLINE_COMP by default for writes like we
already do for reads, and the clear it for all the cases that actually
do need a user context for completions to update the inode size or
record updates to the logical to physical mapping.

I've audited all users of the ->end_io callback, and they only require
user context for I/O that involves unwritten extents, COW, size
extensions, or error handling and all those are still run from workqueue
context.

This restores the behavior of the old pre-iomap direct I/O code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20251113170633.1453259-5-hch@lst.de
Signed-off-by: Christian Brauner <brauner@kernel.org>