bcachefs: Make btree_deadlock_to_text() clearer

btree_deadlock_to_text() searches the list of btree transactions to find
a deadlock - when it finds one it's done; it's not like other *_read()
functions that's printing each object.

Factor out btree_deadlock_to_text() to make this clearer.

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