bcachefs: Mark write locks before taking lock

bcachefs: Mark write locks before taking lock

six locks are unfair: while a thread is blocked trying to take a write
lock, new read locks will fail. The new deadlock cycle detector makes
use of our existing lock tracing, so we need to tell it we're holding a
write lock before we take the lock for it to work correctly.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>