btrfs: tag as unlikely if statements that check for fs in error state

btrfs: tag as unlikely if statements that check for fs in error state

Having the filesystem in an error state, meaning we had a transaction
abort, is unexpected. Mark every check for the error state with the
unlikely annotation to convey that and to allow the compiler to generate
better code.

On x86_64, using gcc 14.2.0-19 from Debian, resulted in a slightly
reduced object size and better code.

Before:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  2008598	 175912	  15592	2200102	 219226	fs/btrfs/btrfs.ko

After:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  2008450	 175912	  15592	2199954	 219192	fs/btrfs/btrfs.ko

Reviewed-by: Anand Jain <asj@kernel.org>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>