btrfs: use the inode item boolean everywhere in overwrite_item()

btrfs: use the inode item boolean everywhere in overwrite_item()

We have this boolean 'inode_item' to tell if we are processing an inode
item key and we use it in a couple of places while in another two places
we open code by checking if the key type matches the inode item type.
Make this consistent and use the boolean everywhere. Also rename it from
'inode_item' to 'is_inode_item', which makes it more clear that it's a
boolean and not an instance of struct btrfs_inode_item, and make it const
too.

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