btrfs: trimming some start_transaction() code away

btrfs: trimming some start_transaction() code away

Orabug: 25615755

Just call kmem_cache_zalloc() instead of calling kmem_cache_alloc().
We're just initializing most fields to 0, false and NULL later on
_anyway_, so to make the code mode readable and potentially gain
a bit of performance (completely untested claim), we should fill our
btrfs_trans_handle with zeros on allocation then just initialize
those five remaining fields (not counting the list_heads) as normal.

Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: Alexandru Moise <00moses.alexander00@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>
(cherry picked from commit f2f767e7345dfe56102d6809f647ba38a238f718)
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
 Conflicts:
	fs/btrfs/transaction.c