btrfs: tag as unlikely error conditions in the transaction commit path

btrfs: tag as unlikely error conditions in the transaction commit path

Errors are unexpected during the transaction commit path, and when they
happen we abort the transaction (by calling cleanup_transaction() under
the label 'cleanup_transaction' in btrfs_commit_transaction()). So mark
every error check in the transaction commit path as unlikely, to hint the
compiler so that it can possibly generate better code, and make it clear
for a reader about being unexpected.

On a x86_84 box using gcc 14.2.0-19 from Debian, this resulted in a slight
reduction of the module's text size.

Before:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1939476	 172568	  15592	2127636	 207714	fs/btrfs/btrfs.ko

After:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1939044	 172568	  15592	2127204	 207564	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>