btrfs: remove redundant zero/NULL initializations in btrfs_alloc_root()

btrfs: remove redundant zero/NULL initializations in btrfs_alloc_root()

We have allocated the root with kzalloc() so all the memory is already
zero initialized, therefore it's redundant to assign 0 and NULL to several
of the root members. Remove all of them except the atomic initializations
since atomic_t is an opaque type and it's not a good practice to assume
its internals.

This slightly reduces the binary size.
With gcc 14.2.0-19 from Debian on x86_64, before this change:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1939404	 162963	  15592	2117959	 205147	fs/btrfs/btrfs.ko

After this change:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1939212	 162963	  15592	2117767	 205087	fs/btrfs/btrfs.ko

Reviewed-by: Qu Wenruo <wqu@suse.com>
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>