Fix error handling in the ENA driver and lock drbr_free() call

Some goto tags were renamed for consistency, and few error handling
routines were reworked.

The drbr_free() must be locked just in case code will change in the
future - for now, it should never be an issue, because drbr is being
flushed in the ena_down() call, and the lock is required only when there
are some mbufs inside.

Submitted by: Michal Krawczyk <mk@semihalf.com>
Reviewed by: byenduri_gmail.com
Obtained from: Semihalf
Sponsored by: Amazon, Inc.
Differential Revision: https://reviews.freebsd.org/D12859