For some reason, the lookup() retry loop (for when happened to
race with a removal and grab an invalid WeakPtr) locked
the lock again. This causes the #3836 crash since the lock
is already locked. It's rare since it requires a lookup between
invalidation of the WeakPtr and removal of the WeakPtr entry.
Fixes: #3836
Backport: bobtail Signed-off-by: Samuel Just <sam.just@inktank.com>