btrfs: mark all error and warning checks as unlikely in btrfs_validate_super()

btrfs: mark all error and warning checks as unlikely in btrfs_validate_super()

When validating a super block, either when mounting or every time we write
a super block to disk, we do many checks for error and warnings and we
don't expect to hit any. So mark each one as unlikely to reflect that and
allow the compiler to potentially generate better code.

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>