This adds a new helper, bch2_trans_run(), that runs a function with a
btree_transaction context but without handling transaction restarts.
We're adding checks for nested transaction restart handling: when an
inner transaction handles a transaction restart it will still have to
return it to the outer transaction, or else assertions will be popped in
the outer transaction.

But some places don't need restart handling at the outer scope, so this
helper does what they need.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>