bcachefs: six locks: Fix lost wakeup

In percpu reader mode, trylock() for read had a lost wakeup: on failure
to get the lock, we may have caused a writer to fail to get the lock,
because we temporarily elevated the reader count.

We need to check for waiters after decrementing the read count - not
before.

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