btrfs: simplify exit paths of btrfs_evict_inode()

Instead of using two labels at btrfs_evict_inode() for exiting depending
on whether we need to delete the inode items and orphan or some error
happened, we can use a single exit label if we initialize the block
reserve to NULL, since btrfs_free_block_rsv() ignores a NULL block reserve
pointer. So just do that. It will also make an upcoming change simpler by
avoiding one extra error label.

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