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>

fs/bcachefs/fs.c

@@ -895,6 +895,16 @@ static int bch2_getattr(struct mnt_idmap *idmap,
 	stat->subvol	= inode->ei_subvol;
 	stat->result_mask |= STATX_SUBVOL;
 
+	if ((request_mask & STATX_DIOALIGN) && S_ISREG(inode->v.i_mode)) {
+		stat->result_mask |= STATX_DIOALIGN;
+		/*
+		 * this is incorrect; we should be tracking this in superblock,
+		 * and checking the alignment of open devices
+		 */
+		stat->dio_mem_align = SECTOR_SIZE;
+		stat->dio_offset_align = block_bytes(c);
+	}
+
 	if (request_mask & STATX_BTIME) {
 		stat->result_mask |= STATX_BTIME;
 		stat->btime = bch2_time_to_timespec(c, inode->ei_inode.bi_otime);