erofs: clarify direct I/O support

Currently, only filesystems backed by block devices support direct I/O.

Also remove the unnecessary strict checks that can be supported with iomap.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20241115074625.2520728-1-hsiangkao@linux.alibaba.com