btrfs: store and use node size in local variable in check_eb_alignment()

Instead of dereferencing fs_info every time we need to access the node
size, store in a local variable to make the code less verbose and avoid
a line split too.

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>