bcachefs: Prevent granting write refs when filesystem is read-only

Fix a shutdown WARNING in bch2_dev_free caused by active write I/O
references (ca->io_ref[WRITE]) on a device being freed.

The problem occurs when:
- The filesystem is marked read-only (BCH_FS_rw clear in c->flags).
- A subsequent operation (e.g., error handling for device removal)
  incorrectly tries to grant write references back to a device.
- During final shutdown, the read-only flag causes the system to skip
  stopping write I/O references (bch2_dev_io_ref_stop(ca, WRITE)).
- The leftover active write reference triggers the WARN_ON in
  bch2_dev_free.

Prevent this by checking if the filesystem is read-only before
attempting to grant write references to a device in the problematic
code path. Ensure consistency between the filesystem state flag
and the device I/O reference state during shutdown.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Author: gShahr

fs/bcachefs/super.c

@@ -1757,7 +1757,8 @@ int bch2_dev_remove(struct bch_fs *c, struct bch_dev *ca, int flags)
 	up_write(&c->state_lock);
 	return 0;
 err:
-	if (ca->mi.state == BCH_MEMBER_STATE_rw &&
+	if (test_bit(BCH_FS_rw, &c->flags) &&
+	    ca->mi.state == BCH_MEMBER_STATE_rw &&
 	    !percpu_ref_is_zero(&ca->io_ref[READ]))
 		__bch2_dev_read_write(c, ca);
 	up_write(&c->state_lock);