btrfs: move unlikely checks around btrfs_is_shutdown() into the helper

btrfs: move unlikely checks around btrfs_is_shutdown() into the helper

Instead of surrounding every caller of btrfs_is_shutdown() with unlikely,
move the unlikely into the helper itself, like we do in other places in
btrfs and is common in the kernel outside btrfs too. Also make the fs_info
argument of btrfs_is_shutdown() const.

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
  1939044	 172568	  15592	2127204	 207564	fs/btrfs/btrfs.ko

After:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1938876	 172568	  15592	2127036	 2074bc	fs/btrfs/btrfs.ko

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>