iomap: Rename IOMAP_ATOMIC -> IOMAP_ATOMIC_HW

iomap: Rename IOMAP_ATOMIC -> IOMAP_ATOMIC_HW

In future xfs will support a SW-based atomic write, so rename
IOMAP_ATOMIC -> IOMAP_ATOMIC_HW to be clear which mode is being used.

Also relocate setting of IOMAP_ATOMIC_HW to the write path in
__iomap_dio_rw(), to be clear that this flag is only relevant to writes.

Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20250303171120.2837067-3-john.g.garry@oracle.com
Signed-off-by: Christian Brauner <brauner@kernel.org>