btrfs: simplify error handling logic for btrfs_link()

Instead of incrementing the inode's link count and refcount early before
adding the link, updating the inode and deleting orphan item, do it after
all those steps succeeded right before calling d_instantiate(). This makes
the error handling logic simpler by avoiding the need for the 'drop_inode'
variable to signal if we need to undo the link count increment and the
inode refcount increase under the 'fail' label.

This also reduces the level of indentation by one, making the code easier
to read.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>