[PATCH 5/8][next] btrfs: Avoid -Wflex-array-member-not-at-end warnings

[Gustavo A. R. Silva]

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Change the type of the middle struct member currently causing trouble
from `struct bio` to `struct bio_hdr`.

We also use `container_of()` whenever we need to retrieve a pointer to
the flexible structure `struct bio`, through which we can access the
flexible-array member in it, if necessary.

With these changes fix 32 of the following warnings:

fs/btrfs/volumes.h:178:20: warning: structure containing a flexible array 
member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavo...@kernel.org>
---
 fs/btrfs/disk-io.c | 4 ++--
 fs/btrfs/volumes.h | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index f09db62e61a1..2fbaaa9ab3e3 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3963,7 +3963,7 @@ static void btrfs_end_empty_barrier(struct bio *bio)
  */
 static void write_dev_flush(struct btrfs_device *device)
 {
-       struct bio *bio = &device->flush_bio;
+       struct bio *bio = container_of(&device->flush_bio, struct bio, __hdr);
 
        device->last_flush_error = BLK_STS_OK;
 
@@ -3982,7 +3982,7 @@ static void write_dev_flush(struct btrfs_device *device)
  */
 static bool wait_dev_flush(struct btrfs_device *device)
 {
-       struct bio *bio = &device->flush_bio;
+       struct bio *bio = container_of(&device->flush_bio, struct bio, __hdr);
 
        if (!test_and_clear_bit(BTRFS_DEV_STATE_FLUSH_SENT, &device->dev_state))
                return false;
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 120f65e21eeb..6eb55103b3d1 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -175,7 +175,7 @@ struct btrfs_device {
        u64 commit_bytes_used;
 
        /* Bio used for flushing device barriers */
-       struct bio flush_bio;
+       struct bio_hdr flush_bio;
        struct completion flush_wait;
 
        /* per-device scrub information */
-- 
2.43.0