bcachefs: Unlock in bch2_trans_begin() if we've held locks more than 10us

bcachefs: Unlock in bch2_trans_begin() if we've held locks more than 10us

We try to ensure we never hold btree locks for too long - bcachefs tries
to be soft realtime. This adds a check when restarting a transaction,
where a transaction restart is cheap - if we've been holding locks for
too long, drop and retake them.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>