btrfs: tag as unlikely branches that call extent_io_tree_panic()

btrfs: tag as unlikely branches that call extent_io_tree_panic()

It's unexpected to ever call extent_io_tree_panic() so surround with
'unlikely' every if statement condition that leads to it, making it
explicit to a reader and to hint the compiler to potentially generate
better code.

On x86_64, using gcc 14.2.0-19 from Debian, this resulted in a slightly
decrease of the btrfs module's text size.

Before:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1999832	 174320	  15592	2189744	 2169b0	fs/btrfs/btrfs.ko

After:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1999768	 174320	  15592	2189680	 216970	fs/btrfs/btrfs.ko

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>