btrfs: tag as unlikely error handling in run_one_delayed_ref()

btrfs: tag as unlikely error handling in run_one_delayed_ref()

We don't expect to get errors unless we have a corrupted fs, bad RAM or a
bug. So tag the error handling as unlikely.

This slightly reduces the module's text size on x86_64 using gcc 14.2.0-19
from Debian.

Before this change:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1939458	 172512	  15592	2127562	 2076ca	fs/btrfs/btrfs.ko

After this change:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1939398	 172512	  15592	2127502	 20768e	fs/btrfs/btrfs.ko

Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>