bcachefs: bch2_check_fix_ptrs() can now repair btree roots

bcachefs: bch2_check_fix_ptrs() can now repair btree roots

This is straightforward enough: check_fix_ptrs() currently only runs
before we go RW, so updating the btree root pointer in c->btree_roots
suffices - it'll be written out in the first journal write we do.

For that, do_bch2_trans_commit_to_journal_replay() now handles
JSET_ENTRY_btree_root entries.

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