btrfs: remove unnecessary extent map initializations

When allocating an extent map, we use kmem_cache_zalloc() which guarantees
the returned memory is initialized to zeroes, therefore it's pointless
to initialize the generation and flags of the extent map to zero again.

Remove those initializations, as they are pointless and slightly increase
the object text size.

Before removing them:

   $ size fs/btrfs/extent_map.o
      text	   data	    bss	    dec	    hex	filename
      9241	    274	     24	   9539	   2543	fs/btrfs/extent_map.o

After removing them:

   $ size fs/btrfs/extent_map.o
      text	   data	    bss	    dec	    hex	filename
      9209	    274	     24	   9507	   2523	fs/btrfs/extent_map.o

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>