bcachefs: support STATX_DIOALIGN for statx file

bcachefs: support STATX_DIOALIGN for statx file

Add support for STATX_DIOALIGN to bcachefs, so that direct I/O alignment
restrictions are exposed to userspace in a generic way.

[Before]
```
./statx_test /mnt/bcachefs/test
statx(/mnt/bcachefs/test) = 0
dio mem align:0
dio offset align:0
```

[After]
```
./statx_test /mnt/bcachefs/test
statx(/mnt/bcachefs/test) = 0
dio mem align:1
dio offset align:512
```

Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>