bcachefs: Fix WARN() in bch2_bkey_pick_read_device()

syzbot discovered that this one is possible: we have pointers, but none
of them are to valid devices.

Reported-by: syzbot+336a6e6a2dbb7d4dba9a@syzkaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/errcode.h

@@ -310,6 +310,7 @@
 	x(BCH_ERR_data_write,		data_write_misaligned)			\
 	x(BCH_ERR_decompress,		data_read)				\
 	x(BCH_ERR_data_read,		no_device_to_read_from)			\
+	x(BCH_ERR_data_read,		no_devices_valid)			\
 	x(BCH_ERR_data_read,		data_read_io_err)			\
 	x(BCH_ERR_data_read,		data_read_csum_err)			\
 	x(BCH_ERR_data_read,		data_read_retry)			\

fs/bcachefs/extents.c

@@ -227,8 +227,11 @@ int bch2_bkey_pick_read_device(struct bch_fs *c, struct bkey_s_c k,
 	if (have_io_errors)
 		return -BCH_ERR_data_read_io_err;
 
-	WARN_ONCE(1, "unhandled error case in %s\n", __func__);
-	return -EINVAL;
+	/*
+	 * If we get here, we have pointers (bkey_ptrs_validate() ensures that),
+	 * but they don't point to valid devices:
+	 */
+	return -BCH_ERR_no_devices_valid;
 }
 
 /* KEY_TYPE_btree_ptr: */