iomap: only follow dio alignment requirement if specified

After commit 4025185cf953 ("iomap: follow the alignment requirement for
iomap_dio_hole_iter()"), generic/091 fails on both xfs and ext4.

The reason is that ext4 and xfs (except COW write) do not require full
block alignment for direct IO. They are totally fine reading a sub-block
range.

That commit fails any direct read of sub-block holes/prealloc, even those
filesystems can handle it without any problem.

Fix the regression by only follow the block alignment if the
IOMAP_DIO_FSBLOCK_ALIGNED flag is set.

Fixes: 4025185cf953 ("iomap: follow the alignment requirement for iomap_dio_hole_iter()")
Reviewed-by: Pankaj Raghav(Samsung) <pankaj.raghav@linux.dev>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Link: https://patch.msgid.link/0e89c49b6f7804081f4041007b81ac620a153830.1793307063.git.wqu@suse.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>