btrfs: unfold transaction abort at btrfs_copy_root()

Instead of having a common btrfs_abort_transaction() call for when any of
the two btrfs_inc_ref() calls fail, move the btrfs_abort_transaction() to
happen immediately after each one of the calls, so that when analyzing a
stack trace with a transaction abort we know which call failed.

Reviewed-by: Daniel Vacek <neelx@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>