bcachefs: Fix check_snapshot_exists() restart handling

Codepaths that create entries in the snapshots btree currently call
bch2_mark_snapshot(), which updates the in-memory snapshot table, before
transaction commit.

This is because bch2_mark_snapshot() is an atomic trigger, run with
btree write locks held, and isn't allowed to fail - but it might need to
reallocate the table, hence we call it early when we're still allowed to
fail.

This is generally harmless - if we fail, we'll have left an entry in the
snapshots table around, but nothing will reference it and it'll get
overwritten if reused by another transaction.

But check_snapshot_exists(), which reconstructs snapshots when the
snapshots btree has been corrupted or lost, was erronously rechecking if
the snapshot exists inside the transaction commit loop - so on
transaction restart (in this case mem_realloced), the second iteration
would return without repairing.

This code needs some cleanup: splitting out a "maybe realloc snapshots
table" helper would have avoided this, that will be in the next patch.

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