bcachefs: Fix missing commit in backpointer to missing target

bcachefs: Fix missing commit in backpointer to missing target

Fsck wants to do transaction commits from an outer context; it may have
other repair to do (i.e. duplicate backpointers).

But when calling backpointer_not_found() from runtime code, i.e. runtime
self healing, we should be doing the commit - the outer context expects
to just be doing lookups.

This fixes bugs where we get stuck spinning, reported as "RCU lock hold
time warnings.

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