ebtables-translate: Use shared ebt_get_current_chain() function

ebtables-translate: Use shared ebt_get_current_chain() function

Drop the local reimplementation. It was barely different enough to
be buggy:

| % ebtables-nft -A foo -o eth0 -j ACCEPT
| % xtables-nft-multi ebtables-translate -A foo -o eth0 -j ACCEPT
| ebtables-translate v1.8.5 (nf_tables): Use -o only in OUTPUT, FORWARD and POSTROUTING chains
| Try `ebtables-translate -h' or 'ebtables-translate --help' for more information.

With this change, output is as expected:

| % xtables-nft-multi ebtables-translate -A foo -o eth0 -j ACCEPT
| nft add rule bridge filter foo oifname "eth0" counter accept

This is roughly the same issue fixed in commit e1ccd979e6849 ("ebtables:
fix over-eager -o checks on custom chains").

Signed-off-by: Phil Sutter <phil@nwl.cc>