bcachefs: Fix assertion popping in transaction commit path

bcachefs: Fix assertion popping in transaction commit path

We can't be holding read locks on btree nodes when we go to take write
locks: this would deadlock if another thread is holding an intent lock
on the node we have a read lock on, and it tries to commit and upgrade
to a write lock.

But instead of triggering an assertion, if this happens we can just
upgrade the read lock to an intent lock.

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